On many foreign carriers, the weight limits for carry-ons are far more stringent and actually enforced.
When flying EasyJet, every carry-on is weighed and sized.
I have even had my carry-on weighed when flying J on AF.
As far as weight limits on UA, I have never seen a carry-on weighed, but on most planes, the weight limit for each section of overhead bins (which I believe measures about 3 rows) is 125-150 lbs.
So I don't think one 100 lb bag would be appropriate as a carry-on.