LogFAQs > #971820457

5 has some minor issues, like the unskippable navigation dialogue and the rng to the final stage opening up (and I think heart tanks only apply to the character who picks them up but it's been a while, although you can get armor pieces with either)
6 is really unpolished and has a lot of poor design choices (like some bits of the final levels you can't get through with certain armors or unarmored or something) that might drive you crazy the longer you try to play it.
7 I haven't played but is pretty unpopular due to the poor attempt to transition to 3d, the new character, and having to unlock X.
8 is often seen as stepping back in the right direction and more positively receive, though it still has a few doofy gimmick levels (one stage is just the boss fight, on the hover bike). I didn't finish it though.

gameplay issues get compounded a bit by story issues because X5 was intended to be the conclusion to the series and set up for the Zero games... but Capcom kept making X games in spite of all that.