One would think the opposite would be true with the whole driver thing on a closed system, but you would know better than me.
For standard controllers, of course. This would include the WiiU Gamepad and WiiU Pro Controller. For last gen and third party hardware? That's where things get tricky.
For example, even though the Guitar Hero Guitar is essentially a Classic Controller with limited buttons and a different form, it is considered it's own device, and Activision had to work with nintendo to make drivers for it. These drivers are loaded in the system updates.
You'd imagine that perhaps the WiiU would have the old Wii device drivers kind of stock, since they seem to still be using the Wii Remote Plus in a lot of games, and the old non-Plus model still works on some games too. This however is hard to say for sure since I don't have access to a kit. And if I did, which I may in the future, I'm quite sure I won't be able to talk about it, lol.
There was actually an article related to this that I read yesterday, or the day before. The developers of Skull Girls made a driver themselves to support PS3 fight sticks on PS4, for their upcoming PS4 release of the game. SOny never provided such backwards compatibility, but they did sign off on the drivers once they were made so they're legit now. What's more is they're giving them out FOR FREE to any developer who wants to use them. How cool is that? Good Guy Lab Zero Games.