I don't remember having any problem with the ninjas, or with any other enemy. Once we get used to their patterns, it's just a matter of timing their attacks to reach them. And all of the boss battles were really fun.
Offscreen attacks would only be cheap if the enemies used more advanced AI and had hitscanning, which is not the case.
I've died a lot, but I don't remember the game having cheap challenges at any point, so I enjoyed it. It's a matter of learning the level layout and the enemies' attack patterns, and trying again.
The only issues were with a few places were we can get locked in with no way out, which Roel already knows.
The only additional suggestion I'd make is that maybe one of the costumes could have significantly stronger directional control during regular jumps, to help in places were we must jump on lots of horizontally moving platforms.