I really feel like a lot of the love this game gets is for not being horrible like X6 and X7. It's close to being decent but there's too many flaws for me to really like it.
Most of the levels are annoying, or not very fun, the only exceptions i can think are the intro stage, dark mantis, antonio and the forest. The stages are too focused on being gimmicky instead of giving us some good fast action plataforming. It wouldn't be that much of a problem if the gimmicks were actually fun like Antonion's stage, but they're not. At least they're not HORRIBLE (except for Manowar's stage, that is one of the worst levels in megaman as a franchise)

The rare metals and the money grind to buy them sucks, and honestly i feel like most of them aren't worth it. I never even used the Subtanks. Zero's weapons are cool but i didn't find any of them on my own and the levels not being fun made me not want to replay them for upgrades. I really wanted to use the knuckles the whole game but i barely did because i found them when i was close to beating the game.

I know the game is like this and very grindy to give you reasons to replay it with more upgrades each time, and i'm not against the idea of having NG+ in a MMX game, but the levels are not fun for me to want to return to.