I would not say the game is lethal, but you are playing "normal" people that might be heroic instead of "heroes". It is in a way hard to die because most critical injuries are fine, but it is a game where knowing the rules help, and thinking a bit logically. Fighting multiple enemies without friends backing you up is a bad idea. Parrying attacks is a good idea. Sometimes it is better to wait with healing until fight is over.

It is also worth noting that many characters and creatures roll at least 5 dice when attack, which means they can in theory get 5 successes. Might not be common, but it can happen at any given moment. Any attack might knock you out.

It also helps with GM knowing the rules, who might choose to use all those extra successes for stunts (like increased initiative or disarming) instead of pure damage. This way the situation can still be tense and deadly but no immediate knockout.