Oh oh, old dogs, new tricks!
Dave, I've been away from it for a couple years, but I assume you're talking about two part adhesive for bonding panels. e.g. Lord Fusor, 3M, etc.
I know the factories are using these systems on new cars, most of which are unibody. That's a key point, because the entire shell is a stressed member to some degree or other. As such, bond integrity is critical to maintain crash resistence. So, in a word, they work great. Many of the OEM's have, to my knowledge, resisted authorizing wide spread use in the autobody repair arena because of a concern for adherence (pun intended) to proper procedures. At the factory they control the variables, temperature, pressure, bead size, placement, etc., to get repeatable results. It's their opinion that not all repair shops are willing to go to that much effort. The adhesive suppliers, of course, believe they have developed procedures that will allow the repair shop to closely duplicate the factory "performance". I would say you should read the material specification and procedures literature from your preferred adhesive supplier to find out how confident they are in performance, and whether or not you can duplicate their recommended procedures.
We used door skin (non stressed panel) adhesive for years. In fact I insisted on it because of the ancillary values. One, it made for a "quieter" door, two, with a full bead of adhesive around the shell flange there was no opportunity for moisture entrapment. I would hate to see a repair come back in a couple years with rust "dust" showing around the skin flange. We never used the material for larger body panels, mainly due to the OEM warnings. By now the technology may be more advanced, or the experience curve is farther along. If your subject repairs are on the older cars with separate bodies and frames, then I think the opportunities would be more favorable. With the exception of convertibles/roadsters, there really aren't any stressed, exterior panels of a critical nature. Depending on which panel it is, and where your cut line is, theoretically you could attach a complete panel with no welding. Again, follow manufacturer (adhesive) recommendations for proper application. As for patch panels I can speculate, though again, the adhesive company tech department can probably give you better answers. I'm not a big fan of putting in patch panels with the flange method. It's easier for the novice who hasn't developed butt welding skills, but it is also more difficult to reshape (due to double thickness), and is a great place for moisture entrapment. With adhesive, you won't have the heat to distort the panel, and the bead would provide a barrier to moisture incursion. A panel seam (at the cut line) e.g. at the quarter panel to roof sail, should probably still be welded for integrity of the filler base sake.
We used mig welding all the time with acceptable results. Of course we weren't doing concours metal work either. The "old boys" prefer gas welding so they can hammer the bead flat (mig beads are pretty brittle, and don't hammer well, partly a heat thing). Tig is supposed to be better as it is less brittle, and you can control heat, thus distortion of the sheet metal, better. Any method should be acceptable depending on what you're best at. Each guy seems to have his own ability to figure out a way to make whichever method he prefers work for him.
Stitch welding is a starting point. The voids between welds could be a source of "movement" or additional flex, eventually damaging the filler and/or finish on top of it.
Check with your welding supplier for "heat dam" materials. We used to use a blue material that looked kinda like paper mache. I'm sure it was some sort of non-asbestos, organic material. You'd wet it, put it near the weld area, and it stopped the heat migration. It was pretty amazing stuff. Sorry, don't remember what it was called. Otherwise, there's the old wet rag routine. Again, I've seen some be successful with it, and others make a choclate mess causing warpage. Practice, practice, practice.
BTW, most geezers won't believe in the panel adhesive thing, so if you're going to try to get opinions from users in the field, you'll probably need to be talking to the younger guys. Change sometimes comes hard for some.