While he is extremely boring on flat ground with no accessible springs or anything, he's got plenty of... dare i say it, tech, that can be used to go through the campaign, almost.
Sunky is the perfect amount of broken to stay challenging while providing a lot of ways to exploit it, mostly with his taunt. in certain scenarios it can be used to airwalk forever, but it requires good timing, and his lack of jump height provides the question of how to get any higher than you currently are, and each time you reset your air walk timer, you have to taunt again, which shoots you down some bit. There's more to the taunt than just that, but it's an example of how Sunky can be used not to troll whomever who plays as him, but as an extremely limited character that makes getting to certain areas all the more rewarding to pull off.
Of course though, some gripes, at least with this mindset/playstyle.
* Sunky is very slow on the ground and it might not always be possible to abuse his mechanics to attain more speed, and while that's a big part of what makes him challenging when in the middle of platforming, it would be nice to not have to wait.
* Castle Eggman 1's Bridge is impossible, it requires speed or height, and you start off too low to have height, and you need to come to a stop before you can make an attempt at it, suggesting you add a way for him to use the still functional death door, he could very possibly beat the game.
* Holding Kick to maintain momentum, while technically functional, it's not very good at it, dramatically slowing him down, so by the offchance you have the space to make use of say, a horizontal spring or gentle slope, it's not easy to stay that way without resorting to not landing at all.
* Hanging animation.
* Having Sunky's z momentum go flat sounds far more original than just making his death sprite the only thing that really happens with it, and while it may be for the best that he not die in a single hit, i feel like that old bit of code could potentially be uncommented, or make dying in one hit a console command.
While this character is slow and enraging trying to play casual, there is undeniably some value here in doing things that Sunky really shouldn't be capable of.