https://www.science.org/content/article/modern-human-females-and-male-neandertals-had-trouble-making-babies-here-s-why

True but those are purely speculative. The science showed that the Neanderthal genes associated with the testes (iirc) reduced fertility in sapiens

I think it is more like tigers and lions. Neanderthal human hybrids were likely less fertile, particularly any hybrid with a Neanderthal y chromosome. This has gone extinct.