In my 1e-based game I use a really simple critical hit system: on a natural 20, roll a d10. On 8-9-0 damage is 2x-3x-4x respectively. It has served me well for many a year. And *nothing* is immune to criticals, provided you can hit it at all.
For fumbles, on a natural 1 roll a d6, on 1 you have fumbled. If your to-hit roll is brought to 1 or less by things that would logically make you more likely to fumble (e.g. bane effects, being badly hurt, shooting into melee, etc.) then 1 on d6 indicates a minor fumble, using a less-dangerous subset of the main fumble table. (note that when a vastly superior fighter is fighting at minuses vs. a weak opponent, it can sometimes happen that a given roll results either in a fumble or a hit depending on the d6!)
Fumbles get more creative than criticals - you can break, drop or throw your weapon; clobber yourself or an ally, usually not too hard but sometimes full damage and even rarely critical; trip or stumble; drop or break a shield, etc.
Lane-"condition critical"-fan