However, subsequent to FB, I independently built the declarative schema management tool skeema.io which is used by several large well-known companies. So Atlas is a competitor, and I may inherently be biased in my skepticism of tools that use DSLs to generically handle multiple DBMS. (It sounds like we are in agreement there though, regarding your comment on FB's all-MySQL environment.)
Anyway, in my previous comment, my point was that it is certainly possible to build trustworthy declarative schema management tools. It's difficult and requires a lot of effort, but it is not impossible.
Facebook didn't limit schema structure too much, btw. One of my main projects there was building the in-house DBaaS interface, which was used for all sorts of things across the company, quite a wide variety of workloads and table designs. iirc the only major table structure limitations were: you must have a primary key; you must not use foreign key constraints; you must use InnoDB (or later MyRocks, but that was after my time). These are fairly common requirements among other large MySQL-backed companies though, definitely not specific to Facebook.