Sorry @SimTrippy but that is not true :( I have played Sims 4 since the beginning (8 years?) and I got two vampire-alien hybrids in a 100 % Vanilla Game.
The first hybrid is still in my building testing save, so I do not care. But the second one was a tragedy: she was born in my 5 years old main save, even after a fix was patched... She was a really, really glitchy sim. To avoid any problems with my save, she died by sun... The tragedy give me a new story for her dad: my vampire was the first sim I use to play the Paranormal SP.
@sadiesue you can do a Save As of your save before your baby before it grows to a toddler, that way if you get a glitchy hybrid you can return the baby to Sixam...