I wouldn't recommend doing it from a cronjob, there are sometimes things that require sudo to update, breaking changes and important notes from Brew maintainers in the output. It's really important to inspect the output in order to see everything Brew doesn't consider normal.
Lately I'm running this like once every 2-3 weeks and it's still surprisingly nice to use. The problems only really manifest when you never run this "maintenance" snippet and only use the `brew install`, which will attempt to do the housekeeping stuff when you want/expect it the least.