I would never label U2 as "art rock" at their core, yes they've dove into it a little here and there, but I don't look at their catalog and think "art rock".
I also wouldn't ever call them "slow and dark". All their albums have some slow songs, and every album has some meloncoly to it, but once again "slow and dark" isn't how I would label any of their albums. Maybe you have different versions than me
Their records are quite varied, but they have more slow/midtempo, dark songs than any other kind. If Achtung Baby isn't slow and dark then I don't know what it is. Zooropa is mostly slow to mid tempo. Same with Joshua Tree, and it is quite a dark record. Pop is half slow, and even the mid tempo/faster songs are dark, at least lyrically. Unfortettable Fire is quite varied, but half the songs are so slow and drifting they barely move. Same with Passengers, and that record is dark. Not black, but blue.
None of their records are black, but I find that they tend towards darker shades overall, ATYCLB and Bomb aside. Hell, even those records are predominantly mid-tempo half ballad things. Thing is, they are pop records with no adventuring, no experimentation, no risks. They are very conservative and traditional, and they have led U2 to the point that Bono sees they have arrived at.
U2 have always made pop records, but they have been adventurous pop records that strive for something more than to provide simple pleasure, and that's what makes them "art rock" to me. A sense of adventure and depth of intention, and the last one had a bit of that, and all of their 80s and 90s records did, and they were never really irrelevant.
I get it if you think they're not an art rock band, but they are - or were - a very adventurous rock band, and when their music takes a conservative turn they lose that special thing, and they become just another band. They lose a bit of their relevance.
