It isn't so much their combat capabilities but their "presence" onscreen seems underwhelming. The 3rd one in the first encounter with them is definitely the hardest to deal with.
RE: Summoners - micro-missile burst quickly deals with them (and most things).
the Hell knights etc dont scare you at all now because this latest DOOM has no atmosphere at all, i.e if you put the Hell Knight into the Evil Within then yes you'd be really scared
in this latest Doom all you do is run off or kill him easily with the rocket launcher etc