That's a shame, that you've stumbled upon so many fics that don't care about characterisation. But unfortunately fanfic can be a crapshoot like that - character personalities being erased for the sake of pretty people being pushed together isn't a flaw that's exclusive to polyshipping. Maybe it would just be harder to ignore in a polyshipping fic? When you've got more personalities to juggle, it might be more obvious when those personalities start feeling off.