D&D 5E Monsters/NPCs: Class features and feats

[TwoSix]

Do people NOT do that? Feats give stuff like that DR, the -5/+10 mechanic, free rerolls on d20 checks, cantrips and rituals on an otherwise melee character, extra hit points, and higher saving throws. That's all stuff I freely give to NPCs to make them interesting, I don't use some kind of algorithm.