Some of the more recent comments have been encouraging enough to put me back on track to trying NIN in Endwalker after originally planning to shelf it, but there's still a few things that I just don't get. I've just started levelling it, but some things already stick out.

Mainly, Phantom Kamaitachi just feels like a complete waste of a skill. At least with the Raiju's it looks like Square was trying for something. Kamaitachi looks like a completely negligible gain. Some quick math looks like you'd end up with you possibly gaining around 130 potency over 90 seconds (although potentially losing 5 ninki to do so). That just seems so worthless, and a waste of a skill slot. Am I missing something here?