They have bad life sustain so are not efficient at capturing camps alone due to the need to constantly hearth and reheal.
Artanis constant trait means he takes no damage once it is procing and all he has to do is spam his dual strike. He is by far the most efficient I find as he can do weaker camps from level 1 and harder ones at level 4.
Samuro actually doesnt take any dmg taking any camps and Greymane can do siege camp or sapper camp without taking any dmg and can do impalers with pretty minimal dmg as well.
Also Samuro can solo bosses if microes well and at 20 you dont wven need to micro at all to take bosses.
If you want to 1 game the quest you will need to do bruisers.
So can Artanis and Gazlowe.
Yeh they are also pretty good at taking camps. Illidan has problems because he is illidan so outside of taking camps it can be a pain to use him against some comps.
varian is the best hero for any camp solo or in groups, double sword and heal on every single hit, by level 7 he can solo boss camps, i prefer to do boss camps at lvl 10 though so i can have a better cooldown with the charge skill.
with the change to sylvanas, she is no longer the greatest low to mid camp capturer, though she can still do camps its just a lot weaker