It's not my business to care why people really hate this game. I mean they'll point out a bunch of things. Probably like bad translation, "bad game design" (LMAO), Hard bosses (that aren't really hard). Maybe its cause Zero doesn't have a saber. Or maybe its cause Kei Inafune wanted to end things at X4. But it didn't happen. I remember someone saying that knowing the history and reasons why the game is made helps with game development in the future. But I don't see how. I don't see how Inafune not wanting to make an X game after X4 but being told to do so affects any future game. I just don't' see that in the games.
I'll tell you what I don't like about this game. It's the rescue Reploids thing. See throughout each level there's a ton of reploids scattered through the stages. You have to touch them to rescue them. However if these Nightmare Bugs get in contact with them first. The reploids turn evil and attack you. From that point you can either kill them or let them escape. The reploids hold abilities, health up and weapon energy ups. So you don't want to lose them. In other words considering where a lot of them are...you have to decide whether you want save them and possibly sacrifice your life. Or let them die.
The other thing is certain stages being a little difficult to progress forcing you to be innovative. Although it's not impossible. It's not like Kaizo Mario where you're forced to use things you didn't even know was in the game. Also this game does help you out from time to time by modding the layout a bit for some stages.
My last big complaint is the lack of enemies. But its not as bad X4, X5, or X8. Sure it's not a huge variety but at least the number is fair and not non-existent.
Everything else I either don't care for or like. Boss designs, music, Armor, abilities, etc.