I wouldn't actually call Mbox a standard. Thunderbird supports it, but I'd argue it sucks as a way to escape Gmail, because no competitor to Gmail supports it as an import method. In fact, even Gmail itself can't import from Mbox. It may be a standard in that it's defined somewhere officially, but it's not really a standard in the definition that it's commonly used.

As someone who has nearly completed their GExit, I'd say I found Takeout to be borderline useless as an export strategy.