Well, in cornsnakes, they'll breed to other rat snakes (same genus, both new world and old world) kingsnakes (different genus). Then there's the other colubrids. I haven't been able to search up any issues. There have been hybrids bred past the f5 generation and there are also three way crosses (king x milk x corn). F1 clutches have the most issues, but what I've found, once you get past F1, there's no issues.