The data, I’ve seen is that the aero savings is between 0.5 and 0.9 watts.

ENVE says they use recessed nipples because it alllows them to design rims where the spoke holes are molded not drilled, and that their design puts less stress on the rim and the spoke for a stronger wheel.

The watt saving is truly deminimis. The alleged strength advantage in my experience is a non issue. I’ve never had a problem with rims cracking or spokes breaking on comparable wheels, such as Zipp that do not use recessed nipples. Also I don’t see why you can’t mold spoke holes and still use conventional nipples.

For me, the theoretical advantages of recessed nipples are imperceptible, while the hassles are very real