Shoulder charge takes time to build up and is close range while running you can't shoot.
Wombo is a grenade and a slow instantly from any range(yes the further you are the harder to hit)and can start shooting the target.
My solution a 1.5 second delay between grenade and melee when using toxic smoke and
Maybe lessen the slow by 10%-20%
It also has a cool down and can get screwed by sprint lock,also shoulder charge like I said you can't shoot you are literally running into someone majority of the time shoulder charge isn't that hard to dodge or gun them down where as wombo is near impossible.Yes wombo uses abilities but that doesn't change the fact it's a 99% of the time a guaranteed kill.