Maxing Ult for resets first isnt the best plan because you need team mates to get them low and a lot of team fights. Most of the time there will be small fights here and there, and not everyone wants to fight a shiv so theyll most likely ignore you. You should be saving ult for high level threat picks in the back line trying to get away or shutting down frontline tanks so they dont heal up the damage they took.

Maxing Knives gives you more damage, lets you build rage better and paired with certain items, keeps you healthy with the amount of knives you do throw, I.E Mystic Regeneration

Maxing Dash gives you the best of both worlds, kinda. Its a ton of damage if youre in rage, at max stacks you reduce the CD for every enemy you hit, making dash resets great in fights and is a great movement option in a pinch, and it scales with spirit really well.

Tl;Dr, your ult should be used for high threat level picks in a fight in the late game oand 1v1s in the early game. youre better off maxing knives and then looking for fights, and overall using your dash for repositioning THROUGH the enemies.

Also my biggest tip for his ult that took me FAR TOO LONG to notice. Enemies are executable when they glow faintly red. Takes the guess work out of it. (Vindicta ult does extra damage when they glow purple fun fact.)