I've been using mailchimp for the first time in the last couple weeks and found it to be a similar experience to squarespace: extremely friendly to non-coders, but kind of buggy and somewhat hard to use beyond their given templates. (Even the design looks very similar)

My specific problem: I fiddled with the "merge fields" (basically handlebars regex-replace tool) and somehow this broke my email template, but only after I had tested it once and it worked properly. When I copied the original (working) "campaign" and re-sent it, all the text of the email was gone. I copied it and fixed the merge fields, then all the images were gone. After reimporting the images it finally worked.

The really frightening part was that it worked the first time but somehow the act of copying the working email with weird merge fields broke a bunch of other things. So I suppose the copying was imperfect and created a problem with their regex-replace logic the second time. Very glad I tested it a second time before sending it out, overall it took me 6 tries to get it working properly.

I'm on the free tier, and customer support is a paid feature, so I had to figure this out myself.