For complicated stuff, describe the wound from the character's perspective, offer an opportunity, and ask: "what do you do?"
Not: "He smashes your arm with the club and your hand flies off."
But: "he smashes your arm with the club--it looks really bad, just flopping around and you can't move it, you can't even feel it anymore. What do you do?" If the player says "I gather it up and get to the Angel," the arm can be saved, obviously. If the player says "Fuck it, I chop that bastard's head off," make a hard move. Buddy can pick his arm up off the floor later.
Not: "You take a bullet through the lung, another through the kidney, few more in the guts. You're down."
But: "You get shot. It hurts, you feel like you're drowning, and there's A LOT of blood. What do you do?" Don't describe the full effects immediately, drag it out, give them the full 4+ harm wound description over the course of several actions. That gives them a chance to deal with the effects, or die heroically.
Especially of you're in the middle of a battle, and unless you've been literally torn apart, you can't just immediately diagnose wounds, other than realizing pain and when parts don't work like normal anymore.
Also, keep in mind that if a PC goes past midnight, they can't be revived, so you can freely narrate any kind of death. If the player doesn't like it, they can trump what you say with their own narration by marking a debility.