I’m not recommending boiling or simmering skulls, this is why
Bone is primarily made of calcium phosphate and collagen. Collagen provides a flexible network for the strong calcium phosphate to bind to, as by itself calcium phosphate is quite brittle.
Collagen starts melting at about 160°F, and the deterioration rapidly increases once you read about 180°F. If you’re boiling/simmering skulls at 212°F, you are absolutely destroying the collagen fibers in the bone, leaving you with a fragile calcium phosphate shell that can easily break, crack, peel, etc.
The other negative is the fact that high temperatures like those involved in simmering and boiling actually force grease deeper into the bone, making degreasing nearly impossible and take much much much longer.
Beetling or macerating does not damage the collagen in the bone, and macerating actually begins degreasing the bone at the same time it defleshes it.
Good luck