I find demoman extremely UP.
...
Which is why nobody plays him.
Demo isn't UP, he's completely dependent on lag and human error.
Demo has a much higher fire rate than soldier and can instakill with a ~3 sticky traps. Perfect aerial detonations do 45 damage.
Demo is hard because you have to aim and detonate to get a hit. The first is okay once you get used to the arc. The second is dependent on lag, and how good you are at predicting lag.
So demo is just much harder to play than the other classes due to the way he does damage. There are certainly people who play him for his unique abilities, especially now that his damage radius is much better.