Some breeds have been bred crest to crest for hundreds of years and have adapted.
There is a known issue with breeding crested SFH to crested SFH - high mortality, vaulted skulls, inability to hatch...
Every breed is different and has evolved differently. In the southern regions of Sweden, farms had mostly uncrested birds. Other regions to the north had primarily crested birds. We can assume a certain amount of evolutions in those regions that kept primarily crested birds... and that birds from regions that kept primarily uncrested birds would likely have different genetics. Thus, it is likely that the most fatal of crosses result from breeding crested birds to heterozygous crested birds with a primarily uncrested lineage who have not had time to evolve to the point of lower mortality rates from homozygous crests.
Long story short... a Swedish Flower Hen is not a Silkie or a Polish. Just because one can breed other breeds to have heterozygous genetics for crests does not mean the same can be applied to a breed where at least half the gene pool is not meant to be crested.