Minix supplies the X8-H, X8-H Plus, U1 and U9 with 3A power adapter because they have more than two USB ports whereas on the X6 with a 2A adapter with only two USB ports. The box itself can function with as little as 1A. The reason why most USB hubs are junk is because the power adapter that comes with them are under wattage for the number of ports. Each USB 2.0 port should be able to handle up to 2.4A or 12W, so two USB ports will require 4.8A or 24W and three USB ports 7.2A or 36W. As you can see even with a 3A power adapter the USB ports are under powered and Minix is only calculating that the USB devices you plug into the USB ports do not consume more the 500mA per port. By proper definition Minix should be supplying a power adapter with 5V 8.0A or 40W.
Anyone that is really concerned about power adapters stated performances should really test their power adapter and chargers to make sure they are up to snuff. Get a tester like the RuiDeng voltage meter and an electronic load to stress the power adapter and phone chargers.
I agree things are much easier when refering power-in powe-out circuits. The device you point out is the only way to know exactly how much juice a device is taking when in action.
I had recently a doubt about charging the PS4 controllers with smart phone chargers, instead of the PS4 itself, and I thought it would be peacefully and universally accepted that any USB charger would do...
Fortunately I was also recently told about users having remotes fried with some smartphone chargers, and that came into mind. Digging deeper I found out that the smart phone chargers that allow a boost charge are not exactly the same as what these chargers used to be any longer, as they allow a charge of either 5v and between 500mA -the actual value for computer USB ports -I think that is a standard, my two laptops have that, and that is probably why Minix ports have that value- to 2000mA (depending on the charger model), up to 9v - and around little less than 2000mA!
There is no warning, but these two voltages are written on the side of the charger, should you have good eyes, or good spectacles, and interest, knowledge, and care to look for this info before connecting any USB device to it, believing it will do the trick.
For some strange reason, when you use one of these chargers that are being shipped with recent expensive phones you will put at risk your 5volts device because it sometimes happens the voltage it needs is not recognized perfectly by the charger, and this has apparently happened already with PS4 controllers.
Of course these controllers won't suffer if you connect them to a under wattage charger, it will take longer to charge and that will be it.
I won't dare to confirm a motherboard with all sorts of micro voltages acting between the several circuits and components can be safe under fed, when not safely protected between the ingoing power and the circuits by capacitors or batteries to level the power to safety zone, as it happens in the Minix as the power goes straight from the PS into the circuit. But I accept that the knowledge I have is old, that is why I doubt myself more and more these days, trying to keep out of trouble.