The .50 BMG uppers have been out for a few years now and there have been no reports of a failure on the lower receiver. All it does is provide the fire control group and no stress is put on it.
There's a few different type of uppers.
Ferret 50 (very accurate and has won many matches)
BOHICA/FAR 50
Watsons Weapons (old design that I would skip)
UltraMag 50
Ultralite 50
ALS (probably the cadillac of them all)
Here's the low down, the ALS is probably the best but the wait time is 18+ months. Ferret 50 is second and the wait time is 6+ months. The rest vary in terms of what they're capable of doing, the BOHICA is the newest to the market but already has a huge following and that has pushed the wait time to about 1 year.
As far as recoil goes, you'll get a little more then a standard .50 rifle because they're lighter then a regular rifle. The other thing to do if buying one is to use a dedicated lower because there's more to them then just slapping one on top of a lower.
Lastly, they're just a part and not a full rifle with serial numbers, so you don't need to go through an FFL to get one.