MJML is easily the best tool of its kind and I use it a lot. If anyone is trying to build emails in 2024, it's a major shortcut that helps avoid and mitigate some of HTML email’s biggest headaches.
Using frameworks like mjml can add a lot of arbitrary markup that’ll rapidly increase the size of your email.
In my experience one of the biggest causes of clipping is styled links with lots of tracking, oddly enough.