Phage,Re lethal traits - there are multiple genes contributing to crest.
The crest interferes with the sutures in the skull and can change the shape of the skull - or even leave a defect (as in silkies). It could be that if too many genetic influences for a large crest could cause a lethal deformity and reduced hatch rate in these chicks.
With swedish flower hens it is il-advised to breed crested to crested as if will produce a huge crest and I believe lead to a reduced hatch rate for this reason. I certainly used to have some dead in shell when I was hatching the swedish flower hens. The same goes for silkies.
That said I have no problem at all getting CL eggs to hatch, and very very rarely have a dead in shell. But then my birds have normal sized crests- not huge or tiny (and I have an idiot-proof bator).
thanks for the insight... someone who hatched from eggs I shipped - had a domed skull female. Another person found incomplete skulls in eggtopsey. I think I may have a female here with recessive domed skull -- that when line bred will produce skull problems -- I guess it is time to check her into the "eggs only" squad and eliminate her from breeding.