Lots of Discussion happens when you talk about mig welding downhill. Personally, I prefer to go uphill on anything 3/16" and thicker. But for open butt root passes in pipe welding in fab shops, downhill mig is pretty common and yields good results provided the bevel, gap, fitup, and land are within scope.
I worked in a pipe fab shop a long time ago and tested on a 8 inch schedule 80 5g pipe...downhill root and uphill fill and cap.
I seem to remember tack welding using tig...then feathering tack welds with a grinder...and then welding half the pipe from top to bottom without stopping.
There are variations of the short circuit mig process where the arc is manipulated electronically to provide even better results on open butt root passes.
Miller has a system called "pipeworxs" that uses something they call Regulated metal deposition (RMD) for downhill roots in pipe.
Lincoln Electric has a process called STT that stands for "surface tension transfer" that is designed for welding downhill root passes in pipe.
Fronius has a process called "cold metal transfer" (CMT) that is designed for specific applications