This is a big reason why I like Datomic so much. I get the benefits of a schema, but the schema is basically append-only, so it's very easy to reason about. Updating the schema is instant.

The last time I tried a project with a no-schema DB, it was a huge PITA. It sucks having a function fail on one single document because it had a number instead of a string. I love flexibility, but it is very valuable to have the right constraints in the right places. I quickly learned that a schema is just that.