Here are a few caveats I've noticed so far:
1. In order to enable or disable, you need to have your seatbelt on, otherwise it is greyed out.
2. Seatbelt also has to be on, while in drive, for the brake hold to activate. Otherwise, there will be a slash through the icon on the panel.
3. This does not work in reverse and is intended to per the manual on the F-150s. I am sure there is a setting that disables this and now my search starts anew.
4. When parked and shifter in park, you place your foot on brake to shift out of park, until you press the gas the hold is activated so long as your seatbelt is on.
I am sure there are more, and I will post as I find them.