any demoman that can make low heal priory situations work are gods, even if they dont get a lot of picks, as long as they can put out a ton of damage for the combo to clean up and at the same time, land pipes in 1v1s when they get focused, they are heros. Just being able to win 1v1's with scouts with pipes is so important.
I would say also just being able to make use of quick arrow heals while letting scouts get more buffs is essential.