I think X&Y suffered from 2 main things, the first thing is that at the time Coldplay were under immense pressure to follow up the massive success of Parachutes and A Rush Of Blood To The Head, when they delayed X&Y iirc their label’s stock price dropped, that was how big the expectation were. From what I read the band argued a lot during the recording and just didn’t enjoy the process of making X&Y much, it’s a more personal dislike around the making of the album rather than a dislike of the album itself, I’ve read a bit about their feelings about X&Y, and it seems like they would want to refine and edit the album better, they do like stuff like Fix You (which makes sense since that song is great). The second issue is where it sits in their discog, it comes after 2 albums that are quite similar in sound, and X&Y does carry a lot of the same aspects (albeit while adding new stuff into the mix) and was succeeded by Viva la Vida or Death and All His Friends which was a massive shake up sonically, and I would argue is even more creative than X&Y in some ways.