5v is only half the story and mostly not the problem.
Sufficient amperage is key here.
Agreed. Voltage regulation and sufficient amperage are very importand. You also need adequate quality cables and connnectors to ensure minimal voltage drops. The problem is typically that USB power supplies that have enough current capability are often compromised in terms of their ability to maintain a good 5 volt supply at the connected device due to the cabling. USB specs allow for some voltage drop, but the short term current spikes required by some platforms such as the Pi can be significant and often result in excessive voltage drops unless the cable is adequately rated. The problem only gets worse with longer cables, of course.