It uses MJML under the hood to ensure email-safe HTML is generated. That should prevent many edge cases where failures can happen - but I'm sure there are some skeletons we'll have to find / fix.
FWIW We're switching to Sparkpost (I'm not affiliated with Sparkpost). They do the transactional email for the big dogs like Twitter, PayPal, LinkedIn, etc and have a pretty simple UI. Just throwing that out there for all the fellow Mandrill refugees.