In my experience what you have to look out for is the instant kills and Mind Shackles.
Losing a 400 point figure to a 90 point Mind Shackle is not awesome.
Worse is the possibility that Braxas could destroy Hulk, Superman and Thor in a single turn.
The last time we played mixed, my Jotun threw Hulk in a lava field for an instant destroy. That was pretty much the end of mixed 'scape at our table.
Classic squads can still be a hassle but at the higher point values they quickly run out of room in a standard 24 hex start zone.