i will add you CAN'T MIG WELD on UHSS, ultra high strength steel. you have to use a squeeze type resistance welder, STRW, that duplicates factory spot welds. this includes welding the UHSS panels themselves and welding panels to it.
i will clarify this a little bit. as of now, you can mig weld the mounting flanges of UHSS. mounting flanges are what's under the weatherstrip. you can weld these areas but you are strongly suggested on using a STRW. if the area is panel bonded from the factory you must use STRW welders in that area. i would assume in the future they will remove this recommendation when there is more crash testing and there are more STRW welders in shops.

you can MIG BRAZE, also know as silicon bronze welding, in limited areas of UHSS. this type of welding is several hundred degrees cooler than mig welding.
to do this type of welding you must buy a welder designed to do it. our shop has a pro spot mig welder. it welds mig, silicon bronze and aluminum. it uses both 75/25 and pure argon shielding gas. have been lead to believe this welder cost around $20K when new. our shop bought this welder about 12 years ago. new machines might be cheaper.

just a fyi. our shop has a car-o-liner Squeeze type resistant welder. i think it was around $30K new. our shop bought it about 12 years ago. new ones might be cheaper now. these welder run on 480 volts. these welders can easily put a spot weld thru 4 layers of sheetmetal with the inner layers of sheetmetal still having the ecoat primer still on it. these machines put out serious magnetic waves when welding, that can stop pace makers and wipe out cell phones a short distance away. i have read about this in training classes but have never experienced it first hand.