The problem is that many USB cables used between a PSU and a device with Micro-USB connector lead to voltage drops. While this is perfectly ok when charging a mobile phone (will take then 12h instead of 8h) it could make the difference on a SBC between 'works' and doesn't work. And the 'funny' thing is: As long as the device doesn't consume that much you won't see the voltage drops (or only slightly decreasing). But they occur when the board needs power (be it an external HDD connected or running under full load).
And the whole discussion is useless since you don't get this unless you visualize the stuff. Take the 5 minutes and install RPi-Monitor with S500 fixes and start to test a bit. Currently the measurement of current/consumption doesn't work since I've no idea which sysfs value to query (or which of them to combine in which way) but at least voltage drops can be measured easily.
Regarding other SBCs that consume a bit more: nearly all of them feature a power jack since otherwise you can not power the board reliably (especially when bus powered peripherals are connected).
I've been able to run LeMaker's Guitar with a PSU rated 4.5V/1A under full load conditions but without any peripherals connected (the power meter showed 6W and the Guitar is different since it feeds the PMU with just 4.28V and they power USB peripherals not through the PMU!). But on the Roseapple PI given the power requirements of connected peripherals (2 x 500mA + 1 x 900mA) it can't work. You need a powered USB3 hub or will get in trouble.