Consider this post by the respected DIY ESC developer Takao Shimizu, talking about a low cost controller available from a popular vendor:
The money quote:
But, when it's over the current (>2A), then the output voltage goes up to the input voltage(11.4V).
Read that carefully: if you overload the ESC (and 2A isn't that much of a load), your 5V supply suddenly becomes an 11.4V (or higher) supply. Think about how much you have invested in your receiver, servos, autopilot, GPS, OSD, camera(s), etc. Many (most) would be destroyed more or less instantly by a surge of that kind.
It's not just power supplies, though. Cheap servos are tempting too, but again they represent a single point of failure for your aircraft. If you're just building a foamie with an anticipated flying life of a few months, you might be willing to accept the occasional failure and early demise, but if you're flying several hundred dollars worth of gear, a few bucks more for decent servos is a much better investment.
