POM/Nanite aren’t very easy to directly compare but my opinion is that Nanite strikes a much better quality/performance balance than POM does.
POMs cost is variable, it depends on the camera angle and how many steps you’re using. Getting quality similar to Nanite requires using PDO with contact shadows which adds some additional cost, and introduces more aliasing into an already heavily aliased effect.
If you’re seriously looking at POM I would suggest you look into Relaxed Cone Step Mapping as a potential alternative. It requires precomputation and additional texture memory but it produces better results at lower step counts.
Fundamentally though since these are material effects you’re going to have material effect problems, parallax won’t look right on the horizon of curved surfaces or along the seams of the sharp edges of a mesh. For this reason, even before Nanite, I was of the opinion that it was best to simply model more geometry into LOD0 instead of relying on POM/RCSM.
As far as Nanite goes, it provides much better quality without the aliasing of POM, LOD popping, and does so while also largely decoupling the rendering cost from the scene complexity which is a huge benefit on its own.
I haven’t yet done much testing with the new Nanite tessellation so I can’t really comment on that. When it was first added I built the engine from source, and the performance cost of the tessellation was pretty steep, especially on landscapes. I don’t know if that has been improved or not.