Buttondown is designed for minimalist newsletters. You get to avoid complex WYSIWYGs and cumbersome blocks and builders. It's a tool that supports Markdown and enhances it even further to allow you to embed tweets, highlight code samples, and more.
And, of course, HTML is valid Markdown! So if you want to really customize your layout, you're more than welcome to.
Buttondown has support for your favorite Markdown extensions (tables, footnotes) and a couple ones I built out just for newsletters.
Buttondown's drafting mode makes it easy for you to view your compiled Markdown as you write, and in your inbox.
Getting dynamic and external content into emails is a pain. Buttondown's rendering extensions make it easy.
Buttondown goes through and makes sure you don't accidentally send out any broken links or typos, too.