This might be important for some:
It costs a monthly subscription and doesn't appear to be open source.
I may give this a spin! I've been using Netlify-CMS for a while, but it's not nearly as full-featured, and seems to have some issues with Metalsmith sites (I've been having difficult getting uploaded files added to the repo).
This is really slick.
I think this paradigm - a CMS that builds out to static files - is really powerful. The notes about speed and scale are spot-on, nothing is reliably faster than just nginx serving flat files.
I'm working on a microblog system based on the same basic idea.
Looks great. Note that the inline demo animations don't work with Mac Safari 11. I had to use Chrome to see them.
Check out Tipe. https://tipe.io.
We have been using datocms heavily at my company for a variety of projects, and have been beyond impressed by how robust and well-thought-out the platform is, and how responsive the staff are to bugs and feature requests. I spend a LOT of time evaluating headless CMS' as part of my job, and Dato is a standout option - one that I recommend as a top choice to anyone who asks, and as an inspiration when I speak with reps from other companies building headless cms'. To be clear I am not employed by or have any partnership with dato at all, just a big fan of their work : )