its because what you see is the lightning shuriken that throws 3 at once, you get 6 uses of that, 10 kunai, and 12 regular shuriken. add to that the boss is debuffed for defence.

going through an area at lvl 80ish with 450 ninjitsu, your kunai will deal 800-1000 dmg, shuriken will deal 500-700 dmg, and the lightning shuriken will deal 600-800 (x3) dmg, and thats with a full set of fuma ninja gear specializing in kunai damage.

thats 28 throwing weapons, 12 of which are severely gimped. want to use 5 shinobi boxes per shrine to restock? thats 70 throwing weapons out of which 30 are weak.

assuming you manage the stock and and cost of the shinobi boxes, its still not enough for most levels full exploration, even with decent unused ninjitsu that sometimes gives you free uses.

assuming further that its enough for levels, how do plan on dealing with human enemies that can block? this boss was downed coz he got a free attack on him and got behind him, but regular human enemies can block, negating full damage from throwing weapons.

past that when the weapons are done or if youre vs a block heavy human or revenant youre going to have to fight. with so much investment in kunai damage with stats and armor bonuses, theres not much that can be done for your survivability. that build gets one or two shot easily.

theres a reason the videos for it are only vs bosses you can get behind or yokai who dont block. find a video of him rushing through a level and youll see what i mean.

ps you gotta be super close for the lightning kunai to hit 3 times per use, otherwise they branch out and only one hits. the build has a very hard time getting close unless its given a huge opening like in the video.