How is Lee harder to master than Yuki??? I understand knowing the setups and crossups for Lee and knowing the resets, but he doesn't have much in combos, there's only 1 or 2 instances in which something can only combo on counter hit. Other than that, if you can out-manuever your opponent and overwhelm him/her, then it shouldn't be too hard imo... His 360 grab does suck in comparison to some of the other characters' 360 throws...
Yuki on the other hand technically only has 2 safe rushdown moves (sans GCFS), and that's 2D (and canceling to IAD into something else) or the first 3 inputs (hits) of Mutation Combination. Other than that, his 2A sucks, his 2B is ok, but not compared to Lee's. Yuki has a limited air game. In fact, he's only good in the air with a timed 9C, or sometimes 9B, but its not like Iori's jumping B, that's for sure. He doesn't have any normal cross ups that don't involve using a special. Lee has I think 3 crossups, and one of those 3 is a command normal. Yuki can do damage, but it involves a charged special, or comboing with a level 3. Lee can simply do a few combos + oki, and it can be over for the opponent. Yuki has better defensive options though, but most of them are situational (i.e.- Sengoku counter, 214A/C, IAD to avoid grounded projectiles, 236A/C).
I feel Ai can be just as hard as Yuki, only because she simply has weird normals, with weird priorities. Same goes for her specials. Alot of them are simply unorthodox, but still have their uses when the setup calls for it. For instance, she's one of the only characters whose projectile can act as a good AA with the right spacing + she can summon tetris and hit it across the screen; I would say Mudman has that same ability, but there are plenty of ways to get around that if you know that he can only have 1 straight, 1 arc, and 1 super out-- the real problem is knowing the order in which the come out and running through them. Her counter is only good for projectiles only, I believe.
Geese, while is one of my favorites in this game, is definitely not the one of the stronger versions of Geese. I feel he's lower, even though he has great normals that can be used to punish, especially standing C. He can do damage, but its only in the corner, and when its midscreen it requires blue canceling at least once (although I think there's one combo he can do that allows oki with reppuuken). Nakoruru is weak, too, but I feel she's stronger than Geese, primarily due to her better combo in the corner that can be added with a 1 stock super, vice Geese, who requires a lvl 3 super to actually combo a super in the corner. Her specials can avoid some projectile spamming, as well as close the gap between her and the opponent. Her command normal actually has good priority, too.