There is no counter for Ferrothorn now unless you run a fire-type dedicated to taking it out. Makes so many Pokemon completely unviable as they have no way to fight it.
I just checked the list on the front page of this thread. Our choices are so limited with Ferrothorn counters. IMO Arcanine/Ninetales are borderline OU tier, but that's just me being hopeful. Charizard seems to be the only good pick to stop Ferrothorn.
Volcarona is gone. Talonflame is gone. Heat Rotom isn't anywhere near OU tier, and using him forces people to give up on the insanely OP Wash Rotom.
Hopefully there are four or five new Fire Poke'mon in Sword/Shield that can take out Ferrothorn. Or some new move that messes him up.