Even RAW it could be argued that an unconscious person is "unaware of being attacked" and thus a valid target of the ambush skill. (While a down, but able to flop around person aware of being attacked would not)
The only problem with jacking things up so much as to add Weapon ranks to Ambush Ranks and apply, is that fate can be gentle. . .like I take a crossbow bolt to the helmet, and drop, knocked out for a few moments amid a battle, you were just fighting with me, and take this opportunity to slash me full OB with your broadsword. . .you could, by chance just happen to hit my big metal belt buckle, or my big holy symbol, or the pouch of coins I'm carrying, and even with your tremendous advantage still just leave a bruise, or perhaps break a bone behind the hard object that stops the edge from cutting into me. It's mid battle, you still need to be aware of people fighting and be ready to defend, so you're not devoting 100% of your attention to killing me.
OTOH after the fight ends, if I'm still laying there bleeding, and you're no longer needing to split your attention, you can likely press your sword into my neck then give it some twists.
It's not like RM combat isn't already quite dangerous when you're standing and defending yourself, and once you have taken a hit bad enough for a down result, usually a follow up attack with unmodified crit is going to be bad enough.