Kog is underused for the same reason Ashe is overused: Utility.
Ashe can dish out CC all day, everyday. She can ult you in the face from across the map if you're careless, and in spite of the long cooldown hawkshot is a great map awareness skill.
Now sure, you can get close to her utility once you can ult bushes and build a mallet... which costs you 3.2k and you have to be L6. Ashe can have slow and map skills by L2 for $0.
Kog is a bit too much of a glass cannon right now. As much as I HATE healing from damage, especially "LOL I outheal your damage", I think Kog may need some of this in his kit. Getting health for dealing damage to champs in his ooze, perhaps. And even then, I'm still not sure I'd pick him over someone like Ashe, or even Teemo.