This is all very true. The performance will be limited by the quality (i.e. max current output) of whatever usb power source you plug into. Some computers only supply high current from specific ports, most android USB charger plugs supply at least 2A at 5 volts but as you say, bumping the voltage up to 9 volts will basically halve that. The standard Apple plug that comes with iPhones is notoriously underpowered so is a non starter. Therefore you’d have to carry your own plug (and, as you say, cable) in order to be sure to power your board properly. At that stage, you may as well just have a conventional pedal power supply and save yourself the hassle.