Why!? I would greatly prefer there to be some definite and unavoidable advantages or disadvantages (or even outright immunities) based on the monster's lore. Maybe slimes are impervious to bladed and piercing weapons, and resistant to bludgeoning? What if golems were resistant based on their composition?
Nothing is stopping you from creating those monsters.
Additionally, monsters have virtually no scary options aside from damage. Level drain is gone, ability damage is insanely rare, aging effects are gone,
And good riddance. Those were seriously un-fun.