So I was going through AoHu (abruptly ended when I realised that even 20 HP and 7 armour is not enough to survive a lava barrel explosion) with my Combat Shotgun (9d3) [5/5] (P2T3) when I realised how absurdly easy the game is once you get a decent combat shotgun and some levels of finesse.
What makes me think that some monsters need natural knockback reduction is my favourite hellgate trick, the one where you hide behind the northern wall and fire/reload/fire/reload so quickly that you can push back both of the bruiser brothers before either can attack you.
In my opinion, bigger and heavier enemies should have much more knockback resistance. Revenants and Hell Knights get about -30%, Barons and Arachnotrons get about -50% and Mancubi/Minibosses get -70% or so.
On the other hand, flying enemies should have about +100% knockback and be buffed in some way to compensate (Cacodemons just get some stat increases, pain elementals that you've injured will spawn lost souls even if you can't see them)