Tom Scott has a video about it (kinda of ) and has some links in it. Ask a mortician has videos on cryonics here and was also interesting.

I'm one of those "got a life gathering facts and topics at random and connecting dots on the fly" and not someone that does science for a living. I know you asked for books, but I can't remember (or link) anything right now.

Size. Mostly.

One of the problems of freezing and adult is that you can not "flash freeze/unfreeze" it. The speed of the process (and medium used) matters because the cells are suffering from lack of oxygen. The slower And the water content inside the process, the bigger the damage. And there is also the water crystals that form during freezing. Thet damage the cells too (by rupturing structures) and one of the ways to prevent it is to freeze the cells super fast, forming less/smaller crystals. Unfreezing may be a problem too, since the temperature needed to heat your core organs fast enough would burn out your skin, muscles and outer layers.

An embryo is a really smal cluster of cells. You can diffuse heat a lot faster since there are less to freeze An adult is a really big cluster of cells.