[Shujin]

I think Hayabusa set is very powerful and does alot, but they are not exactly the "Strongest" dagger set out there. Depending on what you actually want to do. They are however the most fun and versatile set.

You can do alot of shit with other sets, and damage wise Hayabusa is more on the low end. Its super fun to play around with though thanks to the teleport, but everyone who actually uses them and fights against a speedy person, knows that you won't hit anyone from the 7 tiles range with that sweet farshot penalty ontop of your Dagger hit chance (the hit check comes before the teleport so no guile flanking bonus). and against Boxers you are basically doomed to almost always fear the Geist.

It does however put you in a great position against tanks, so they are super useful against those. Combined with burn you also shred a good chunk of their defense which makes your damage comparable to other daggers again but it still won't win a damage race against Fans or even fangfare or Stingers. And the Silence is nice to have even with its pseudo-immunity it applies.

It does have answers to alot of different playstyles just by existing and doing it passivly, thats probably the main problem. Being able to swap damage types is also very useful.
If I were to nerf it, I'd probably start with the Status effects rather than the teleport. Make them Infliciton based, maybe have it a building up status effect like:
1. crit of Tsukikage->Spelldaze, if enemy has spelldaze already, consume it and turn into silence. Or something like that. So it needs two crits to apply.

Burn I don't know, Hikage with its str scaling really sucks, and mundane only raises it to normal dagger levels for an enchantment slot. I think thats fine, honestly. To use daggers effectively is already dooming you to be one rounded in any teamfight, it only really excels in duels, which I think very much fits "assassins". And even then we have a couple of more threatening things outthere, that can kite you, while doing half the things Hayabusa does, without putting you in the center of attention. Teleporting behind someone IS a downside in a team fight, half of the time, as you get focused to death that very instance. (Many people do not realize it, but you put yourself in the perfect position to give the enemy full Flanking bonus against yourself. This is horrible in both PvP and PvE)

Or I don't know, remove Silence and replace it with something more watery. Soaked has the issue of Burn basically instantly deleting it, so I dislike that one.