The differences in the reciprocal crosses is most likely due to the different reproductive strategies of both species.
A Tigon has been successfully bred with a Tiger to produce a “Ti-Tigon”. This Ti-Tigon could re-merge with a Tiger population, bringing a certain % of Lion ancestry to that population.
Similarly, our hypothetical “Humanderthal” could re-merge with a human population, bringing a certain % of Neanderthal ancestry to that population.
Pretty interesting but...
“It’s not nice to fool with Mother Nature”.
One of these days someone’s going to figure out how to cross a mosquito and a condor. Just try spraying Raid on that thing.