I think the main problem is that each league's number of ranks increases considerably, which increases the grind. Sure, if you're winning consistently it's not TOO bad, and if you reach the top 8 you won't have to do league matches ever again for that league (you can do the tournament repeatedly with no loss in rank), but still.
Weirdly enough, while the league examiners have horrible voice direction and volume balancing and the random trainer dialogue whenever Shadow Mewtwo comes up in discussion is kinda mediocre, and Nia is cringeworthy... Once you're really thrust into the game's main plot after reaching A rank, everyone but Nia suddenly has pretty decent voice acting. I'm not even kidding, the game's voice acting quality is practically schizophrenic.
Anyhow, gameplay is pretty fun so far, but probably the two biggest problems with the gameplay is that you're vulnerable just after recovering from a knockdown (the invincibility period ends just before you're even able to do anything to defend yourself from further attack, it's a small window but I've seen AIs exploit it), and combos do far too much damage despite the lack of a mechanic to get out of a combo. An enemy who manages to start a combo can wipe off a big chunk of your HP, then punish you just for recovering from knockdown and damage you even more, it's really unfair.