We're really excited about MJML. Creating messages that render properly on all email clients is challenging and MJML solves the problem in a very developer friendly way. You should expect MJML and the Passport editor to be integrated into Mailgun in the near future!
I always was intrigued by the premise of MJML but it's just a little weird at this point. Client support is inconsistent. I'd rather it less feature rich than having certain components that aren't fully supported by popular clients. IMO it would be better to have a really simple/easy to remember set of components that were fully supported.
I would say it does have that set of simple components: wrapper, section, column, image, button and text are all you really need. These are the ones we set new developers up with. You can create fully client-compatible versions the more advanced components with these components.
Everything else they provide is just gravy for situations where development speed is more important than client support (as we see it) or when you know you’re not going to worry about Outlook users of a certain generation.
All I'm saying is if I wanted to look up a bunch of support charts, I may as well try my luck with regular HTML, because it's no longer speeding up development trying to remember which component is supported where. And the way it's presented in the link above is awful.
Well, now you’ve read my comment and know the reliable ones. I hadn’t seen that chart so I just tested the first emails I built in Litmus and opened them in a few of my own clients. After the first couple, it was clear how to build the rest of them without worrying about compatibility.