I don't think it does as long as it's not overdone. There are already subclasses that gain these sorts of benefits, though they most often have a duration or some sort of requirement attached.

Think Barbarian's Rage or Drakewarden's 7th level feature.

Quote from Llethander >>

I don't think it does as long as it's not overdone. There are already subclasses that gain these sorts of benefits, though they most often have a duration or some sort of requirement attached.

Think Barbarian's Rage or Drakewarden's 7th level feature.

 So maybe durational immunities?

Aye. Duration and Resource cost are both ways of balancing by limiting how often a feature can be used and how long it will last. This also doubles as a means of making the player(s) think more tactically about when to use them.

Then you can also consider what it's going to take to activate. Bonus Action abilities tend to be a bit more forgiving since if it's tied to your Action you likely won't be able to Attack or cast any big, meaningful spells that round.

PCs get immunities and resistances from their race: elves are immune to sleep, dwarves are resistant to poison.  Many subclasses offer resistance to one or two types of elemental damage, occasionally turning it into immunity as a capstone.  Look at what's already available, don't give out something more than that and you ought to be reasonably balanced.

And there are ways to bypass damage resistances: the Elemental Adept feat, for example.  But if you give resistance-bypassing attacks to NPCs, save it for special ones.  It should be significant that an enemy can get through a PC's resistance.  Having every Curly, Moe, and Larry able to bypass it just means that they're not being allowed to use a class/race feature that they chose for a reason, which really sucks for the player.

Quote from Llethander >>

Aye. Duration and Resource cost are both ways of balancing by limiting how often a feature can be used and how long it will last. This also doubles as a means of making the player(s) think more tactically about when to use them.

Then you can also consider what it's going to take to activate. Bonus Action abilities tend to be a bit more forgiving since if it's tied to your Action you likely won't be able to Attack or cast any big, meaningful spells that round.

 Thank you this is helpful

Either would only make things worse if the entire party had the same ones, for the same reason that a party made up entirely of goliath hexadins with identical builds would get old quickly. If a monster or NPC spellcaster or whatnot has a bunch of fire based attacks then it's time for the tiefling in the party to shine because they have an advantage. If all the PCs are tieflings then the entire encounter is empirically less difficult and the DM just rebalances it by adding something else to make up for the fact that everybody is only taking half damage. When everybody's the same kind of special that's just moving the goalposts for normal. 

Resistances are like cinnamon. A tiny little pinch makes a very nice and flavourful dish. Much more than that and it becomes a nasty one that, once tried, no one will ever want to have it again. It also needs to be complimentary to the dish - nobody likes it in their carbonara, trust me (ok, I was still learning to cook, experimenting and didn't realise that mixed spice had cinnamon in it, ok?).

I would be extremely reluctant to give a PC an immunity to anything other than very niche conditions. It's just too powerful, and makes situations just plain boring.

Two quick questions before answering, one what's the point and/or theme of the class, two would the immunities option also apply to Conditions (Frightened, Charmed, etcetera) because in my opinion Condition immunities are far more acceptable.

As with most things, resistances and immunities are fine in moderation.

Damage resistances should be kept to one or maybe two at most on a sub-class, especially as they may be adding on top of a resistance from race (fire for a Tiefling, any elemental for a Dragonborn etc.). Damage immunity should either be a high level feature, or only temporary using a combination of duration and resource cost.

For example, the new chromatic dragonborn have resistance to their ancestor's damage type, but also Chromatic Ward which they can use to become immune, but only for one minute, once per day, and it takes your action to activate, making it something you must use in exactly the right moment. I've used it on a black dragonborn of mine to turn a high level vitriolic sphere into a defensive area of effect by making him immune then nuking the whole room around himself (as a sorcerer he could do this in one turn by quickening the spell).

Condition resistances and immunities are a bit more flexible; immunity to sleep is fine at a low level, but resistance to others is probably more appropriate early on (i.e- advantage on the saves against them). Immunity to a condition I would expect to come in around mid-levels somewhere, whereas immunity to a damage type would be like 15+.

For whether or not these things make gameplay less fun; the answer is "it depends" (which isn't on your poll 😝). A non-excessive number of resistances or immunity features means you maybe sometimes have a player or two who is less vulnerable to certain damage or effects, which can be good for making them suddenly be the lynchpin in the party's response. The key thing for keeping it fun is to make that the exception not the rule; i.e- don't keep throwing enemies with the same elemental damage at your party, give them a variety of enemies so that some are harder, some easier depending upon what the party can do.