Goliath is slow and has to be down in the kill zone, even if it's a player with enough stamina and skill where they can effectively smash in/fire breath, leap out, rock throw, and repeat. First two stages are pretty rough in my experience, but that was against a decent team. Same team against my Stage 3 was a rather different story. Essentially, Goliath has to hit hard and know when to draw back and reapproach.
Kraken might not hit as hard per hit (except that lighting, damn), but it can hit fast and maneuver at the same time. In short, it can multi-task easily while maintaining speed.
I don't think I want to see Kraken be nerfed like a lot of people seem to, but I do think there's a gulf of advantage between it and the Goliath that sort of makes the Goliath a moot 'class' once Kraken is unlocked.