Here are things to take into consideration …
The MiaB backup encompasses EVERYTHING to get your mail server running on a different VPS so if you had some critical incident, you’d be up and running again in an hour. The maildir folders are just that, the mail… additionally, the MiaB backups are encrypted while a straight backup of the maildir directories wouldn’t be. I don’t know if your proposed solution does any encryption itself or not.
If you do the MiaB backup route … be sure to save the secret_key.txt file SOMEWHERE safe, but obviously not with the backup itself.