If a "midcore" player is not able to clear more content than a "casual" player, how are they even different categories? It's just distinction without difference. The problem is these labels are usually used to differentiate people's ability to spend time. But the ability to spend time says nothing about people's skill. The "midcore" players are galore in Bozja, having spent a lot of time amassing enough mettle to get all 30 rays, and yet Red Choctober STILL wipes whole dungeons regularly. And let's not even start about the "midcores" facing the earth / fire combo in CLL. Because that is also usually cleared by only a fraction of all participating players.
So yes, at the very least, Unreals and Extremes (certainly the early ones in an expansion) are midcore. Savages can be more debatable, but at least this tier's first turn should be doable for any midcore player.