I've hosted this software server (with and without mods) for friends and family a few times. Its pretty solid and the mods help bring additional fun or to fill in the gaps of low population. If you love WotLK, i'd recommend it.
A sibling commenter quipped about which quests are bugged and which aren't, but the reality of it is that the vast majority of quests work perfectly fine, including the quests that are heavily scripted (such as the Battle for the Undercity)
The question is more what's bugged and what's not
The largest issue I can think of that actually impacts overall gameplay is the thread system [0]. It definitely works, but currently threat at times isn't given or reduced in the correct amounts. The most common way this manifests is Growl from Hunter pets not properly taking aggression away
[0] - https://github.com/azerothcore/azerothcore-wotlk/issues/5985