Sadly I've never used MacDown due to not having a Mac but did come across it on my search for a good editor.
While none of these options have a Steem specific rendered preview, you can easily work in one program then transfer your content to Steemit for posting. To do this you simply need to know what HTML tags Steemit supports and which ones it doesnt. Here is a post that shows most of the HMTL tags that steemit supports: https://steemit.com/steemit/@vaibhavshah/html-tags-to-improve-your-steemit-articles
By making use of these tags and avoiding ones not supported by Steemit, you can simply copy and paste your work with no formatting errors. Personally, I prefer Haroopad as it formatting is a lot like steemit and it being a great application.