The first two raids in the game in 2007, Helegrod and Rift, are filled with landscape mobs - Angmarim, spiders, drakes, giants, spirits, trolls, hillmen, worms - all identical to landscape versions. Even many of the bosses were just slightly larger or tougher landscape mobs - merrevail, gortheryg, wraiths.
Putting tougher versions of landscape mobs in an instance or raid is not a matter of laziness or inadequate staffing; it's a narrative choice. Instances in this game are narratively linked to the areas in which they're found, so of course they're going to have the same kinds of enemies.
Nice, a defender.
Rift still had greatly differing and unique NPCs, with special animations (Ever-seer and Thraulach). Even the giants weren't just reskins, they had some unique animations and particle effects, rather than just a re-colouring.
Helegrod...hmmm....Thorog was slightly different to other mobs. Spider-wing also had unique spider models, including the boss, all of them having a special attack animation they sometimes used.
Moria...Watcher was entirely unique. DN had a ton of unique mobs made, and they even had insane voice-acting to boot.
Mirkwood...we're starting to see some copy-pasting, but they went out of their way in other departments, the lore and stories were top notch so this one should get a pass. And lieutenant's mount was entirely unique.
I'm not gonna list further, I've provided enough examples. Even if we give in to your argument, this got ripped out completely recently and was replaced by copy-pasted seethed mess.
And thanks for picking into the least influential remark I made! Shows us a thing or two about people who are still defending this mess.