I don't think Mass has anything to do with it. Your mass is the same no matter what you're riding, so the force is only increased by the acceleration. Sure, you can do 60 easily on a moto, but they also stop very well from that speed, whereas most mopeds I see folks riding barely can stop from 30 mpn to make a light.
I spend a lot of time riding through Manhattan, all traffic travels at 30 mph, very heavy and very unruly.
On the stock moped, it's all about rhythm and getting through traffic at the best speed you can so that you aren't a speed bump. This can get dicey. I've had handlebars squeezed between cars on the bridge. Straight horror.
On the Moto you can get out of the way faster, from a stop or rolling. And also, even 160mm+ drum brakes can lock up the tires of a 220lb enduro faster than a moped can.
I will agree, if you are not a very proficient rider, stock mopeds are best. You can't get yourself into the same trouble if you lose control at 25 vs. 55. That said, I think equipment quality and advantages give motos definite advantage, and that's not even considering newer bike with disk brakes and ABS, etc.