Gravity.

"Everyone is entitled to their own opinions, but they are not entitled to their own facts."

That's a lot of liquid metal for those sizes.

The small ones use a ceramic cup to retain the metal and form a shoulder; not sure what they do with the sizes you are looking at, but even with the ceramic cup there are vent holes all around to relieve the heated gases that the shoulder material will soon occupy - I wonder if those same holes in a large size would allow the liquified portion of the stud to leak out in an unpleasant manner.

Overhead, the cup retains the material and straight down the metal has symmetric loading.

I have seen a LOT of those studs being welded installed, from M5 all the way to the larger sizes for steel/concrete constructions (with M20 being a very common size, which is comparable to 3/4"), and I have never seen any other position than PA (1G ?).
I wouldn't like to shoot them overhead, there are quite some projections flying in all directions, with you as the primary target.
I imagine PC (2G ?) position will suffer from asymmetric welds, possibly fusion-related or incompetely filled weld problems at the top, as IM suggests.

Thanks everyone for your responses. Forgot about gravity. AWS doesn't have any prohibitions and is somewhat vague about the testing requirements for the larger sizes.

3DDave - your observations about the ferrule make sense regarding overhead vs vertical.