I think the Psuke is too powerful in terms of plot hand-waving, ad hoc dues ex machina, unless its explained much more later on. I think as it stands, invisible limitless power with no drawbacks seems silly. On that note, Kellhus should seem silly too, but I've accepted him.
As for lost, I meant that no one has the faculty to perpetuate. I believe Meppa is something of a mystery to himself, and more so to those around him. Incapable of teaching it, even if he wanted to. This is probably a bias perspective.
Also, again, I feel like there has to be more to limit the Psuke, and having Kellhus learn it seems like way too much. Even if he could/did, he wouldn't teach it to anyone else. Kellhus can't see the Psuke the same as everyone else. He wouldn't introduce an unknown into the equation.
Maybe he would give it to one person as an elaborate plot, somehow able to transfer the knowledge without being able to wield it, and making sure that said person was unable to themselves teach others.
There really are just a ton of ways it could be used, but I think the theories get too elaborate and too contrived, so at this point I've personally dismissed the possibilities of it, for my own sanity.