You can build kind of blocks that you use, but for anything more complex you're pretty much forced to use a kind of free HTML field, which of course is just a text field where you either suffer by editing html and their templating system in a browser text field (the templating is something Django compatible) or you copy and paste from your text editor which is also a form of torture. I've edited the wrong template on occasion, and saved it. Even if you just stick to their wysiwyg you still have to sometimes add conditional blocks and this also is for me anyway difficult. They have developer mode too, which is your email as a giant yaml.
So they support a bunch of things, but personally I would not use it for anything except simple marketing campaigns. We do use it for that, but someone had the idea of having all customer emails go through it, and I don't really like it.