There is another problem with with wire-fed welding processes on the root pass on pipe-pipe welds in particular, and piping systems in general that has not been brought up yet.
GTAW manually feeds a stiff wire into the melted puddle, so there is (seldom) any wire "stub" left sticking out of the weld. Those that do occur (wire stub or electrode sticking) are very visible, are ground off, and the welder restarts - usually with a freshly ground electrode.
When the root gap on a pipe-pipe weld prep or pipe-fitting weld prep is larger than the wire dia - and this will almost almost always be the case - then the initial "feed" of the wire can get aimed into that root gap. The wire goes into the gap, the welder slightly moves has hand or position just a little bit, and the arc starts up. But the first 1/2, 1, or 1-1/2 inches of wire are already into the inside of the pipe before the arc melts off the wire and the puddle starts up back in the prep area between the two pipes. The stub length of wire is left inside as the start of a plug, or flow trap, or OFD material inside the pipe invisible to the inspectors and cleaners later on.
If EVERY weld and EVERY arc start point is begun by "aiming" the wire directly into the angled wall of the pipe prep area, then this wire stub is immediately melted when the air starts and there is no remnant left inside the pipe. But that degree of care in starting of every weld arc is very hard to verify.
If you begin this weld process, boroscope every pipe after fabrication of the next piece and before flushing and QA to look the stubs and wire traps inside every pipe. Removal of the stubs inside, and of the broken off stubs and grinding grit after they get ground off inside can be very difficult.
If you do the root pass with manual GTAW, then fill out the welds with wire feed, no wire stubs can be created inside the pipe, but then you need a slower root pass.