IMO it's more that Magma Mines and Slime Pits are significantly harder than the rest of the hard dungeons (not considering special quests)
Agreed, but they're also definitely sub-vicious (again, not considering special quests). The problem is more that they share the category with levels like the Rock Garden which are definitely not
and that the random bosses are terribly unbalanced (Grimm's Grotto Bleaty or Doubledoom Bleaty + Tower of Goo are on the harder end of hard, Grimm's Grotto Longdall or Doubledoom Longdall + Medusa aren't that much harder than the easy dungeons if you know what you're doing).
Again, agreed, although I think that Tomithy, Medusa, and Bleaty are the only three that are out of the ballpark.
On the other hand, some of those "easy" levels were none too easy. There's one quest with full-power Bleaty as the boss... don't care that you're a warlord, that isn't "easy".