Have a lot of back and forth on this. Need some more clarification.

Ex: I cast banishment on a creature that has a different home plane of existence (so they go back to their home plane)
Q: Are they incapacitated (the wording on it is kinda weird)
Q2: Can they planeshift back?

In this case they are not incapacitated. Exactly what happens on their home plane is not specified, so it’s up to the DM. It seems they could plane shift back, assuming they have the proper tuning fork, can cast the spell and have it prepared, etc. 

If there are a native of the plane you are on, that’s when they are incapacitated. For example, some random orc, who was born on the world you are adventuring on gets banished, they are incapacitated in a Demiplane.

Also “native” of the plane isn’t defined, I think most people go with born there as the definition. Though immigration policies may vary from one game to another.

Ok cool.
Not being incapacitated is unfortunate.
My argument is that when I use this spell I want the 10 free turns for my team to kill other enemies and prep for their return.
If the DM can just say, no they plane shift back the next round, they had all necessary preparations to do so...
It feels kind of sucky for me. My characters only purpose is to reduce enemy load with debuffs and banishment/polymorph.
If stronger enemies come along and can just come back on their turn I feel useless.
At that point polymorph is infinitely better because I can make them a worm or something and keep them down for an hour.

As a 7th level spell, the odds that you'll encounter something capable of casting Plane Shift innately, or having a tuning fork, is pretty darn low. If you do encounter such a being, just Banish it again and revel in the knowledge that it just burnt a high level resource that it probably only had one of.

Encountering Shapeshifters who are immune to Polymorph is vastly more common.

There's always going to be a chance you'll run into enemies that are sort of your Achilles' heel. Every type of character has that. It's arguably a good way to keep things fresh for the DM to - in moderation - play into this. Some critters have defenses against spells, some (a lot) can't be reasoned with, some need to be engaged from range, others in melee, some will use traps everywhere, some will be considered influential members of the community in good standing, and so on. And in some cases this won't be readily apparent until you confront them or use the biggest stick in your arsenal and find out it wasn't very effective. That's part of the game. 

Yup, I had an NPC try it a couple of months ago on the team tank. He made his save, though....

However, I did this to that character because his player rarely shows up, and I was running the character as an NPC anyway, so nobody would have been stuck "not playing" for the duration until the enemy's concentration was broken. It would have made my life easier, actually, if he had failed the save -- I'd have had 1 less NPC to deal with.

Something worth pointing out with Plane Shift - if the target does attempt it. They specify a general area to return to and they could end up near - but they could end up far. It is by no means a guaranteed way of getting back into the fight - in fact - I would say it's incredibly unlikely to get the target back into the fight.