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.
On the one hand, there are many mbox variations and you may encounter some difficulty in getting your emails out of the Takeout mbox file.
On the other hand, please understand that there is no better option. Said differently: Google Takeout has made the BEST choice here. I giggled with delight when I saw they chose mbox.
And it's pretty easy. I have a few accounts on Thunderbird and I can drag and drop e-mail directly from one account to another, I think they are .eml files. You can also have local accounts just for storage.
As someone who has nearly completed their GExit, I'd say I found Takeout to be borderline useless as an export strategy.