Actually there was some unique content there, though it's been a long while so I don't recall the specifics, and, as importantly as unique content (especially since your colleage cazador has just posted something that could be read as saying Fel players specifically do not want unique content), it was for a good long while the best way to get high-end magic weapons, of the pre-AoS variety.
Surely you mean the "Unique" content were the puzzle chests, & the various mobs you could only find there?
These things still exist, but they don't offer any rewards you can't get easier somewhere else.
as a matter of fact, it's actually harder to efficiently farm in Khaldun then in other pre-revamped dungeons even still.
1. Fel only
2. you can't res in khaldun, you need to go outside.
3. the revenant spawns to fight players that kill other players, since AoS it has been broken. (10 years)
Btw The Ancient Lich hasn't spawned since the Ricardo event ended. (3+ years ago)
Zealots of Khaldun have dropped the same loot as a Bone Knight, or Skeletal Mage since AoS. (10 years)
Pre AoS, it was a good place to farm for vanquishing weapons. Khaldun died when AoS came out.
The "best" place Pre-AoS, was in Ilshenar>Exodus Dungeon>
Gargoyle Destroyers. for invulnerability plate armor & vanquishing weapons. (if I had any screenshots left I'd be-able to prove it)
obviously it's all word of mouth right now though

.
What caused Khaldun to "Fail" ?
My guess, would because it has nothing unique to offer over any other dungeons. I wouldn't say it failed because it is a fel-only dungeon.
Though, based on what you said at the end of your post suggests Khaldun failed because it was in Fel.
And it was just a great, great dungeon that largely was wasted, due to popular lack of interest in the Facet it was placed in.
-Galen's player
I don't speak for all fel players. but IMO it's the lack of Facet-Unique-Content that draws more attention away from fel for everyone, Tram & Fel players alike.