My Jekyll posts are mostly Markdown with some YAML frontmatter, but some have a few "include" statements here and there that use Liquid. The frontmatter stuff is easy enough to transform with some `sed` commands or whatever.

The bigger problem is the surrounding stuff that relies more on Liquid, like email signup forms, the list of posts, the list of tags, etc. All the little things that would need a small bit of work to reformat to work with another tool, it all adds up. And there were some oddities with how the two parsers handled newlines around Liquid, so sometimes Eleventy's output would be jumbled together when Jekyll's looks fine.