Saved by NPC encounters. Yay or nay? How to handle

Future first time dm here. I'm setting up a homebrew campaign for a group of friends for a game that will start later this year. I've noticed in my planned encounters I have a recurring trend. Putting the party in encounters that they are not able to win unless they run, and having them be saved by an npc important to the story.

The first one I have planned introduces one of the big bads of the whole campaign. The party will do something that makes him mad very early on. He finds them shortly after to seek revenge. This is to have take seriously the big bad's faction who at this point will have only been portrayed as bumbling thugs. Its also to show the party that in certain areas, a specific type of magic is highly frowned upon by this faction so they will need to be careful to avoid their attention.

The other one comes much later in the campaign. More of an important side quest. The party takes on a chain of quests to eliminate shadows holed up in caves on a mountain. They realize too late that they are led by much bigger intelligent shadows and there are tons of them. A messenger of this world's god of light, the main god, shows up to aid them, mostly helping their escape. He explains that their deeds so far have impressed the god but they will have to come back later when they are stronger as even he is not enough help to achieve this now.

How do you guys feel about this kind of thing? I don't want the players to feel like they're in a no win encounter. But I still feel like for some specific story beats, this can work. Like they won't be useless in these encounters, they can hurt the enemy, definitely take out the lesser enemies. 

Do you guys think I should avoid this kind of thing? If so how then would you advise me to go about this?

First question, how far are you planning ahead? Chances are, a lot of the encounters you're planning won't even be used if you set things up for more than a few session away. Lay the groundwork of your plans, but don't get attached. The game is incredibly expansive and adaptive, and making a railroad-type experience only increases the chances of deviation. I suggest letting the players lead the way for a more natural experience.

Probably fine only to do it once, and even then, let there be multiple ways out. The second time, players will start to feel disgruntled and that they have no control. I've had first-time players drop out of games very quickly because they didn't experience the degrees of variety that I drew them in with in the first few sessions, because they were the first few sessions. Many of the same story tricks don't work well more than a few times. Instead, leave questions on the table that the players have to seek answers to, to help them feel proactive.

The first one I described with the faction leader will happen in either session 2 or 3 I imagine. It will be unavoidable unless the party decides to not help an innocent family getting killed by other members in this faction in session 1. Possible, but doubt it. The faction leader does give them the option to join his faction to make up for the killed members. Or they can be taken to jail where this faction, which is very anti magic, has ways to strip them of their magic powers. I'll never actually take away their magic but I would have a way to temporarily nullify it just for a session or two. But basically they will most likely want to fight.

Other than this, I'm not 100% sure about the second encounter I described. I might be able to work that in another way. Like perhaps an escape encounter which will lead them to the God's messenger I described. Not sure just yet.

I appreciate the advice. I do plan a lot in advance for main story beats. But I also plan for at least one, sometimes two other possible choices or paths for each one. So I freaking hope I'm doing good lol.

I'd advise against any situation of forcing the players into a combat they cannot win. It is fine for there to be combats they cannot win in the game, but it should always be their choice to take those on or not. If the dungeon full of shadows is too hard for them, why send them there? If they choose to explore it at a too low level that's fine, but I would not send them there as the only thing they can do to progress the story.

Having NPCs available to help the players is fine, but similar to the above there should be some degree of choice in the matter rather than it just being something that happens. For instance with the Cleric of Light - he might be locked in a room or caught in a trap and if they free him he will advise them to leave / help them. For the gang, having an NPC guard currently investigating the gang join them in the fight is fine as long as that NPC doesn't have plot armour and can die if they don't work together with him. 

Well, if players are meant to run away, have the wizards familiar, the druids pet, and any sidekicks tagging along, all immediately suggest the party beat feet. Players generally assume the dm built balanced encounters they have at least some chance of winning. So "run away" is usually not something they consider.

I have had my players fight an encounter and find the bad guys have some massive monster in their lair. Something way out of their league. But they find this after a long combat they won, and the big monster nearly immediately escapes. They run into the monster a couple levels later and by that time the players leveled up and it was a fair fight.

If you can, id say have the monsters retreat at the end of the fight, so the players get a victory and a hook for a future fight. Otherwise the players are left with a fight and retreat which doesnt feel as good.

And also i wouldnt put too many levels between first encounter/retreat and rematch for the actual battle..if they run into something at level 5, say, id have them find it and fight it by level 8 maybe. 

First off, don’t assume you know what the players will do. They will often surprise you. Which means, don’t design an encounter with one correct way to overcome the challenge. They might not realize what it is, and then you tpk. That’s a video game mentality, where you can restart from your last save until you figure it out.
And don’t ever count on them retreating. Players, in general, will not have their characters retreat from an encounter.

An NPC saving them isn’t good either. The PCs are the heroes of the story. It’s their job to save others, not to be saved. This sometimes happens when a DM is really excited about their badass NPC and wants to show them off — it’s a little like the DM trying to be the main character. That’s not your job as DM; you’re there to let the PCs be the main characters. Make your badass NPCs, by all means. But have them off saving other people, then run into the PCs to join forces, but keep the focus on the PCs. 
The party should be Luke blowing up the Death Star, not the other members of red squadron who were nearby when the hero won the day.

Having an NPC save the PCs in a set-piece unwinnable fight is usually a poor plan. (This is different from bailing the players out when you miscalculate or they get in over their heads. And trust me, they can tell the difference.) It tends to make players feel like they're just playing pieces in a predetermined story the DM is telling.

I will echo what has been said, that having an NPC win the fight for the party is often a feels-bad kind of ending. One of the main issues with this is that, by introducing a character who is more powerful than the party, you are undermining their position as heroes. "only you can save the world" said the literal immortal flying soldier of a god who was sent from the heavens to save the party from some trolls.

The bigger issue, though, is one of Autonomy and Agency. This is the player's ability to choose what they do, and expect consequences. If you're reliant on the party failing to introduce this messenger of the gods to save them, then you are inclined to A) prevent them from winning, and B) prevent them from escaping. As combats tend to be the longest part of the game, you'll have the party investing time and energy into a fight which they were never going to be able to win. They'll likely feel a little bad about that.

Now, there's still a way you can offer help without overruling them. Have the messenger be a tiny creature, ill-suited to combat, who gifts the party with some magic to get them through the fight, but makes it clear that was a one-time thing. Make the master shadow one flee, for a later fight without the deus ex machina, and the party can finish off the shadows with their own abilities and attacks, augmented with the light from the messenger, who then talks to them afterwards. Make the NPC kind of feeble but helpful, so they're saying something like "If we can't seal this cave, we have to be away from here before night or they'll come after us!". Make them give the party a sense of urgency and a chance to finish the fight, rather than doing it for them. Make it clear that, without the party, the NPC would outright get squished, so the party still feels like the heroes.

The Player Agency is a tougher one to keep aligned. I would say that having an NPC jump into a deadly encounter and turn the tides as part of the plan regardless of how well the party is doing is fine. Having the fight be unwinnable, then having the NPC solo the enemies, that's the wrong way. You might have a few players cast Area of Effect damage spells and see the Shadows just die, one after the other, or start running. The party might win the fight. Don't lock your plans in on what the party does. Instead, write the past - "The messenger will travel to find the party when they reach the cave, arriving 1 minute after they enter. If they see the fight, they will help as much as they can."

Thus, the player agency remains the same. If they see the cave and nope out of it, then the messenger still comes to see them outside. 

I would generally avoid this. I think this can be a great way to open a story/campaign. Like, your inciting incident is that the party faces a BBEG and is at the end of their ropes when someone comes in to save them, then their campaign is about finding a way to defeat the bad guy. That can work really well, because part of the arc of a D&D campaign is the party growing stronger. 

But otherwise, I'm on board with what other people have said. As a player, once you're into the campaign it's not a lot of fun to have NPC's bail you out of fights that you lost. Mmmmmaybe as a one-time thing, but certainly not on the regular, and even then I'd be skeptical. That's more something for if things have gone wrong (you misjudged an encounter, the dice turned against everyone, etc.), so you can bail the party out. (Edited)

My advice would be to use story elements to change the terms of combat. For example, if the players are going to encounter a banshee, give them the chance to craft earplugs along the way. If they're fighting a master shadow, give them a way to create sunshine. If the bad guy has minions, let them find information they can use to talk the minions into changing sides. Things like that. If you want the bad guy to be an ongoing threat, you can have it retreat from the battle once it starts losing. Or it can always deliver an evil speech then vanish to let its minions handle the dirty work (classics are classics for a reason). You can also build in story elements that give the players a chance to retreat. A key word that can freeze the enemy, but just for one round. A one-use stone of teleportation. Things like that, which are clearly signposted, so when the fight goes wrong they can decide to retreat and fight another day.

However you do it, I would have it be based on player choice, discovery and agency. Good luck!