Good question!

And this comes to the difference between Explore phase and Expand phase.

In the Explore phase, data migration was just running code on the production server via a REPL. Some migrations such as adding/removing fields are just a matter of hot-reloading code in CL, so there weren't a lot of migrations that we had to manually run.

In the Expand phase, once you add replication, this does become hard and we did roll out our own migration framework. But by this point we already have a lot of code built out, so we weren't going to replace it with a transactional database.

Essentially, we optimized for the Explore phase, and "dealt with the consequences" in the Expand phase (but the consequences aren't as bad as you might think).