Just one thought, if you are using a buck regulator or the UBEC, that should be where you get the 5 volts for everything that can use it directly.
The Adafruit text seems clear - that module's logic wants 5 volts Vcc, and (but) its power section can handle 12 volts V+.
Using the UBEC 5 volts eliminates any waste from the Arduino on board regulator, which would be worse if you used the 5 volt out on that to power the servo board.
And it gives a degree of separation between the battery power line, which might have some trash on it from running the servos. A solvable problem, and steps should be taken to clean up the battery power lines, but the UBEC will make nice 5 volts for you.
And I will say that Lipos are an amazing source of power. I use them one cell plus a boost regulator, or batteries of two and more with a buck regulator.
a7