Funcom said somewhere that all places should be seen as end-game zones.
If you want to play in the deadly swamp of no return, zombieland or kingsmouth, then you should.
I would guess that when we talk about: non soloable content, i think their will be obvious indications like Draculas castle, or Mothmans cave.
But about the uglyness i hope not. Would hate to see one of my classmates run around as a boss!!!
But their might be some logic to it (which they seem to love), meaning, you acually have a chance take out a slow zombie alone. But going up against that burning scarecrow with a chainsaw alone, is might not the best idea you could have