ninja mask is really bad for a legendary. It gives 1 haste and 1 weak every turn. This effect will sometimes be applied after other armor has blocked so it only gives its buff to Sheilds. Not that bad right? But if you think about it, it's worse than giving haste to a sheild at a forge which is free. If you were to find a ninja mask at a shop it would cost 35 coins. And it also adds weak and that's bad enough.
In my opinion making this item a rare and making it give 2 haste instead of one. That would fix it in my opinion