MIG welding is easy, GOOD MIG welding is very tough. I had to certify welding pipeline back in the late 60's, underwater 140' deep (dry) with MIG and it was one SOB to do with Xray quality.

On the other hand I've found welding thin metal such as exhaust systems on a car is a snap with a good MIG machine. I would think that getting a decent weld on a bike frame wouldn't t be too difficult but the thing to beware of is even penetration. A MIG weld can look great but really suck for strength if it ain't good.