Valley of Defilement is infinitely worse than blighttown for sure. You can barely even see in the Valley. Blighttowns layout can actually be really interesting if tackled coming down from the depths. The verticality is a nice touch imo and makes both the upper and lower parts of the area feel distinct. Upper blighttown is genuinely tense at times due to the cramped area and the toxic dart dudes. What's neat is that since the dart dudes don't respawn the area doesn't feel unfair. If you can kill one of those guys you're still making progress even if you die. Upper blighttown is of course better than the boring flat swamp of lower blighttown and neither are particularly great areas but I think the hatred is overblown.

Lost Izalith/demon ruins meanwhile is a giant disappointment that just keeps going and going. I'd be less miffed if there weren't so many goddamn bosses. Ceaseless, firesage, centipede demon and bed of chaos. None of which are fun to me or engaging. Compare that to the usual 2(or 1) bosses for all the other areas in the second half and its easy to see why its a drag. It's a lazy, garbage area and I always dread going there. Even exploring it isn't fun. It's just a bunch of randomly pasted rehased minibosses in demon ruins and then more garbage in izalith proper.