I like them because they integrate quite nicely with their other product, React Email [1], where our devs can just write emails in React and it'll render to email-compliant HTML and CSS. I suppose you guys have a GUI as well but I believe they're looking to add that too.
You can upload MJML if you’d really like to, along with designing in Figma via an integration, but really your best bet is our editor.
If we can save a few engineering hours just by having the product person that needs to update the emails with new copy actually update the copy instead of pinging engineering, we think we’ve improved things.
If a competitor ends up building a GUI, glad to see they agree too :)
Just FYI since I happened to be looking into this recently as well :)
[0] https://www.npmjs.com/package/@react-email/render
[1] https://github.com/resendlabs/resend-node/blob/90a7225c21db9...