Some silkies have an extreme vault in their skull, which can lead to brain damage if they bump their head as the vault means all there is is skin over that section of the brain, not bone. That being said, there are many, many breeders of Silkies whose birds have enough skull bone that you don't have to deal with this problem.
I generally am against cross breeding birds, and would advise against this. If you want good silkies, find a reputable breeder and tell them you want sturdy, hardy birds who won't have this problem. Most breeders do breed against this characteristic, fwiw.