When a powered USB hub is used, would we need to worry the power flowing from the hub back into the MacBook or iMac? If I use a non-powered USB hub, the iPad Air or iPad Pro could be losing power over a few hours, because the charging power is not enough.
However, if I get a powered USB hub, would it be an issue, say, if the USB port on the MacBook, MacBook Air, iMac is at 5.2V, while the powered USB hub happens to be 5.3V, in which case the current could be going back into the Mac instead of out of it?
This may be more of an issue if the iPad Air and iPad Pro is unplugged, so the current has no where else to go.
There's an isolation on the power connectors to prevent power from the hub flowing into the USB-A port on a computer.  People have been plugging powered USB hubs into laptops for something like two decades, if this was an issue someone would have seen it by now and fixed it.