The Ferret upper I have is a single shot.
I too was concerned about how the AR lower would respond to the unusual recoil characteristics of a 50, but apparently there is no concern. Talking with a number of guys in the US that have them and have shot thousands of rounds through them , they apparently work very well. I have not shot mine enough to really consider my experience conclusive, but trust my US sources as I know them well.
By putting a 50 cal upper onto an AR lower, needless to say makes the rifle a "restricted" firearm.
We are working on a lower project that hopefully will be un-ARish enough for Ottawa to deem them a Non restricted firearm.
There are a couple of side loading repeaters up here as well, I have 1 here from a client for the very purpose of making a lower that is non restricted.