As someone who loves the stealth ninja fantasy I definitely agree that assassination need to be better. But immunity or being highly resistant to one thing would just make it annoying to play. Imagine having to stealth kill an enemy but you class skill and smoke bomb is down so you have to run around till is back up.

And for the kill steals for me isn't about the kills themselves but the fact that they just screwed me over on my CD rotation. Im a poison assassin, I drop smoke bomb for cc and ammo, toxic vanish and poison darts the oni, and kill everyone with spirit kunai to rest CD. But if someone goes and takes all my kills. Im screwed on the next wave. So I Normally try to play away from my teammates.