There is a paper that was describing, that Neanderthals had their mtdna replaced by something mtdna that was more related to Homo Sapiens starting about 400 000 years ago and fully replaced? 200 000 years ago, so matrilineally Neanderthals were close to Homo Sapiens already. And that question should have not been raised nowadays at all. Besides we also have already found remains of 50% and 25% hybrids, so doesn't seem to be a problem of interbreeding with other humans - just like our ancestors had not issues interbreed with Denisovans as well, with whom we had slightly larger DNA differences than with Neanderthals.

Bigger issue seems to be that Neanderthal inbreeding between themselves as they were living without access to other pools of dna of humans and that most probably was a cause why they did not bounce back.

Rh incompatibility results in limited ability to crossbreed, not complete inability. It usually doesn't affect the first child, only subsequent ones.