but I've heard reports of boards frying with as little as 12V
OK, Vin goes to regulator and you can put 20V on it, is there a Vu pin, it should rout to usb 5V on the board? google the pinout...
There is a (Schottky) diode feeding "VDD5V" from "VDDUSB" on the above schematic but the CH340 USB interface chip is supplied by "VDDUSB", so guess what happens when you provide more than 6 V to this terminal?
I read that Vin pin can take more voltage looking at a bogus pinout image, sorry
Inputs can be driven from either 3.3-V or 5-Vdevices. This feature allows the use of this device as a translator in a mixed 3.3-V/5-V system environment.