Having no cooldown wouldn't help since they're still being checked server side and lag would still force you to wait until you input next or randomly fail, see any 2 off GCD abilities put together, as well as off GCD+instant skills sometimes failing and such. The solution would have been to make mudras client side only, so they wouldn't be affected by lag. Then ninjutsu is checked server side as usual to prevent cheats, it's not like casting mudras does anything until you ninjutsu anyway.

Not a big deal but it's pretty annoying at times, as lag spikes will generally mean a failed ninjutsu. This isn't really new either, there's always been issues in this game with off GCD abilities, for example you cast Swiftcast then Rez(or whatever else), and sometimes you will actually start casting the Res even though you gained the swiftcast buff, sometimes you'll start casting then it'll cancel the cast because Swiftcast fired "at the same time", and sometimes it works as you'd expect.