Some companies do not allow MIG for weld-o-lets. Hard to get full penetration.
Some companies (or at least the local inspector) do not "allow" MIG, but if you look in their standards, it is not uncommon to find MIG allowed for root passes, or for complete welds up to 3/8" thick.
It is interesting that the same companies who do not like MIG in plant piping do not seem to have an issue with fully automatic, narrow groove MIG welds on pipelines.
Most should not have any issue with FCAW.
Some process specifications do not permit the use of GMAW in the short-circuiting transfer mode.
Some will permit it only for joining relatively thin sections.
ASME Section IX mentions the following arc transfer modes for GMAW, short-circuiting, globular, spray, and pulsating.
Try the following link to the National Board Web site for an interesting article by Mike Houle about GMAW-S (short-circuiting) transfer:
http://www.nationalboard.org/Classics/classic5.html