Too many bosses have too many moves that can't be parried for parry to be viable.
Doryani, Tor Gul, the Colossus and more seem to have no moves whatsoever that can be parried, and still other bosses have only a tiny few. Building around parry just isn't practical with the game designed this way. At the very least, we should be able to pick up passives/keystones on the tree that let us parry slams, aoes etc. Or am I mistaken?