The problem with adding randomness to an encounter is that too much of it can start to stray into artificial difficulty territory if certain combinations/timings of mechanics end up being much rougher to deal with.

Having played WoW in its earlier days, some of its mechanics could vary between being either trivial or hellish depending when/where they decide to occur and/or who they target. They eventually learned their lesson and largely stopped having unpleasant scenarios such as both healers or tanks being temporarily out of commission or otherwise placed into a situation that limits or prevents them from executing their role