How effective are outdoor foam faucet covers to prevent freezing?
I live in Canada and there are only two ways to stop your spigot from freezing in the winter.   The first is to have a cut off inside your house, lower then the spigot and it has to have a relief valve on the exterior side to drain the water away from the wall.   The other way is a frost spigot that extends about 20" into the warm zone on the inside of the wall.   This actually has the shut-off all the way into the house.   These have to be installed with a slope towards the faucet end so when you close it the water drains out the spigot.

Those are the only two ways to truly and confidently stop your hose spigots from freezing.