The modern SAAS versions all work without internet using the client app. As we are in Vietnam, I don't think any business would ever choose a POS solution that didn't work without internet or via battery power during an outage. So there's no loss of functionality there. There are power outages around once a month, scheduled or unscheduled, not to mention storm season which has more frequent outages, sometimes for several days. So this functionality gets well tested.

All banking solutions - as well as the POS system - can work on mobile data and that usually is fine during an outage. The only time mobile data failed in recent years was after a 3 day power outage following a typhoon, when I guess their batteries failed. By that time our business was pretty much shut down due to supply issues anyway.

So basically, as long as the battery or generator lasts, all of these POS solutions will perform equivalently.

Edit: and to further clarify, I don't think there are any features that the old schools apps have that the new ones don't. Unless you consider a local database or not using a web browser as features (which is valid but not my view). While the newer ones tend to have a much stronger focus on accessibility (probably because they are basically web apps) and translation.