This forum is dedicated to the Live Steam Hobbyist Community.
With the right wire and proper procedure I wouldn't be scared of a MIG welded boiler, but the person doing the welding needs to know what they are doing. I've used start/stop tabs before, and also ground out a little pocket to start welding into when welding a tube into a hole in a plate. Then you weld over it a bit once you've welded around the tube.
Whatever the process it should never be done with inferior equipment! I hate going behind someone because they thought it would work ok. We all started and learned through weld time and instruction from others. I've helped several who did all the prep work and didn't trust their own skills. As far as mig, dual shield is a good cap after a tig root and hot pass. Pay close attention to what Marty and a few others say. I laugh when I'm told what I can and can't do by those who haven't done. Education is great but experience is priceless!