With large tables, you can't safely make data changes using migration tools either anyway. If you run a single UPDATE against a large table, you end up with a very long transaction with substantial MVCC implications (old row versions that the db needs to clean up) and can basically break production easily.

Side note, but why do folks always bring up this "firstname lastname" example? It is not ever possible to implement that correctly in an automated fashion: some people have spaces in their first names, and some people have spaces in their last names. (I don't mean to single you out, as this exact example comes up multiple times in every declarative vs imperative discussion!)