We now have a reasonably complete list of what is still TODO in Leanpub's Markua support here:

https://leanpub.com/markua/read#leanpub-auto-todo-in-leanpub

This is still a pretty long list, so the default to create Leanpub books is still Leanpub Flavoured Markdown for now, which is what has been used to create books on Leanpub for about 6 years.

Once our Markua support is more complete, we'll document it in our help pages and make it the default.