I've been through projects that gave schema migrations and when there was any mistake or error during schema migrations it was a real big pain to rollback, fix and release. With mongodb this is gone, schemas are no more of an issue as long as you make sure to keep the models sane and the business logic of the fields up to date.
Schemaless means anything can come put of the DB, or be put onto it. It adds a lot of work to the app
developer to keep things validated and secure and there are more exceptions to handle. Definitely a trade off. Never had big issues migrating schema-ed databases.
