This thread was helpful to me while trying to combine the Feather 32u4 Bluefruit with an DFPlayer Mini. I spent a while trying to figure out why the serial connection wasn't working with this board as it was on an Uno running at 5v even though the DFPlayer Mini claims to run on 3.3 - 5v.
Determined that the Mini just does not run correctly on the 3.3v out from the Feather.
It was confusing because it sort of worked, some of the time.
Powering it from the 3.7v battery's voltage pin gave it enough juice to run properly and made all the difference. (VBAT in this diagram: Adafruit Learning System).
I read the original post many times in which the author mentions using the battery's voltage pin and blithely ignoring that detail before stumbling on the same solution, so repeating it here in hopes it helps someone else.
Cheers! 