I think that most of the manaless champions are balanced at the moment, especially if they have an alternate resource that they use.
Yasuo pays for his manaless-ness with a weak early game and an okay midgame. He only has conditional mobility and Wind Wall has a long cooldown, so he still has to be careful with what he does.
Rumble has heat, and while heat management is probably easier than it should be, it does gate him to a degree. Mages can cast-cast-cast in teamfights, while Rumble only gets a certain number before he overheats. Again, he has something to manage.
Tryndamere is resourceless, but also suffers in teamfights due to his inability to stick to priority targets and his general squishy nature. Built full damage, Trynd is forced to use ult early in fights, which forces him into a splitpush role - and that has counterplay.
Gnar is balanced now that his Mega form lasts a shorter amount of time. While he can be devastating in a teamfight if he hulks out at the right time, it's tricky to keep him a hair-trigger from transformation. If he does hulk out too early, his team loses the potential for a fight and has to back off, lest the enemy team engage on them while Gnar is as squishy as the carries.
Riven is obnoxious to play against, and perhaps slightly too strong. However, her stun has a low range and she generally exists as a pure damage dealer due to how the scaling on her shield works. Q's long cooldown also hurts her in fights, since it's most of her damage outside of AAs.
Manaless champions can feel frustrating to play against, since they don't have MUCH to manage in terms of ability costs, but they're far from broken.