As an FGN Master both deepsea and offshore ATBs, whilst technically innovative, are an abonmimation built to have reduced crewing.
My understanding (and correct me if wrong) is that the GRT of the tug section determines the manning of the unit, not the whole combination.
From my PoV this leads to crew overload as they are running the tug at sea then working the brage alongside.
ATBs only exist inthe USA and are built to circumvent the USCG interpretations of manning rules, leading to potentially unseaworthy vessels.
i.e. if the tug could drop off the barge like a semi trailer, go pick up another one then that would make logistical sense. However, as I understand it, the tug section on its own has very little stability making this uncommon.
This isn’t an anti USA rant, just an observation from many years at sea that owners will do anything to bend the rukes to their advantage.
Another example were the turret deck ships frrom the 1890’s. The reduced beam at main deck level lessened Suez Canal fees as they were charged, at the time, on main deck area. The down side was stability had te be carefully monitored.