Currently, monoliths have about 3, maybe 4 different mobs per run. This is despite their length. This leads to 2 big flaws
It makes them boring.
It makes them way too…volatile I guess? Problem is the elemental damage mobs were scaled dramatically with current patches. The void damage mobs were not. A monolith populated with void mobs is mostly joke. A complete cakewalk. Meanwhile, a monolith populated with exclusively end game mobs is a bullet hell with off-screen fast screen covering projectiles that deal heavy damage. The difference is way too great.
It really should be like arena when it comes to randomization.