I just finished Sekiro (it's why I've been posting so much 😂) and been watching a ton of videos on it. But no one ever uses the mortal blade, they always use fire crackers.

But from my experience the mortal blade does high damage, high posture reduction, and staggers with a long and wide reach in one or 2 moves. But everyone I see uses the fire crackers which are immediate and open an enemy up but you need to be close and still mostly only hit their vitality.

For instance, in the 2 headless ape fight all the videos use fire crackers which don't always work. I ran around slashing with my mortal blade which made the second ape pretty trivial and died quickly.

If it wasn't a limited use item (at least in the sense it's only that effective when you have spirit emblems).

Once I got the mortal blade, except for specific boss types (like ones you need the umbrella or divine abduction to get a Puppeteer Ninjutsu mid battle) I kind of found no use for the other prosthetics bc the blade did it all.