I agree with Fealthas.
In the hands of a player that knows what they are doing they are terrifying for lighter units to deal with. Ever had a Jugglenaut jump into the middle of your shield ball and start tossing units around? I have and it can destroy thousands of metal worth of units in the blink of an eye.
There's also a pull / push move that it can do where it can fling units right out of the map. I've had my comm 1 shotted a few times by such BS. Another useful thing it does is act as a damage sink for smaller units to hide behind. A sumo can take bites out of the enemy and then retreat to heal up, and repeat the process.
They are, however, tricky to use and it takes practise. I'm still learning.