It won't be completely stable, when used without a case. The geometry is such that on the side of the lens it's the lens that is touching the surface where you put it, not the bar-shaped bump. Meaning, it won't sit on the long edge of the bar.

I've got an iPhone XR with a chunky (highly protective) Speck Presidio case that's comfortably thicker than the camera protrusion and therefore lays 100% flat.

Gotta say it would drive me nuts to have a phone that didn't lay flat and couldn't therefore be put down safely on the edge of the sink etc.

Apple is always one step back, two steps forwards.