Kamui is an extremely powerful ability, there is no way around it.
You can phase through attacks that would normally kill anyone, and you can do it for 5 minutes.
With ranged kamui you can remove peoples body parts from their bodies, making it an extremely powerful offensive move that's almost impossible to block.
It is good for both offensive and defensive capabilities. It also excels in the ability to spy, run, or capture enemies. It is a ridiculously strong power from pretty much anyway you look at it.
If you are saying that using it is cowardly, I wouldn't think it is. That's like saying any uchiha using any of their ms or rinnegan abilities is cowardly. They are just extremely strong moves. Just because you use a strong move, that doesn't mean using it is cowardly. To be fair it is a move that was acquired through complete luck and is overpowered without training. If anything I guess you could call it unfair or cheap.
