Nick Heer blogging about the Ghost 6.0 release:

If Ghost added MarsEdit support, I would be awful tempted to switch from WordPress.

Probably not going to happen. People have asked for it. Ghost has oddly never cared about open APIs until recently, with ActivityPub, and even that was years after everyone else added support for it.

If you want the most support for lots of APIs and publishing from different apps, there are only two suitable platforms: WordPress and Micro.blog.