Well there had to of been very little difference genetically to interbreed with neanderthals. Look at how close some animal species are and they only create sterile hybrids.
I was looking at this the other day; the dromedary (the one-humped camel, which we generally call the camel) and the Bactrian camel (the two-humped camel) have been successfully continually hybridized over the centuries:
https://www.google.com/search?q=camel+hybrids