Before I purchased my tig welder, I welded some of the smaller Chrome moly tubing on my car with a mig welder. It depends on how good you are with a mig. If you don't get good penetration and the weld is just on top then it is not going to hold. I welded the removable steering wheel lug on the steering shaft and I welded the bungs in the lower control arms on the front suspension with a mig, when I was building my car 12 years ago. I have not had any issues with either.
To be safer on the bungs for the wheelie bars, you can drill a couple of holes in the side of the tubing and do the rossette welds along with the weld around the bung to ensure the holding.
It worked for me, but now I would only use the TIG.
Good luck,
Bill M