Here's some of the stuff I've loaded onto my handlebars while riding (and we're talking vintage road quill stem and 25mm bars which are weaker than fat MTB stems/bars)
-people sitting on them.
-multiple grocery bags (about 20lb/10kg each side).
-40lb/20kg bag of cement ridden 5 miles.
-A 80lb/40kg Pioneer SX1980 receiver (the biggest baddest vintage receiver ever made and found on the sidewalk).
so yeah, stems and bars are pretty strong.