Oh wow! I've used them off and on, and haven't really had many issues. Albeit I haven't run into any issues that warranted contacting support, so I have no idea what that's like.

That said, you don't have to use Mailjet the service to use MJML the templating language. I've also used their WYSIWYG editor quite a few times to edit templates and export the HTML and MJML files for use elsewhere.