He's got a 15% health based damage steroid on any ability he uses after 3 hits.
Vayne's is 8% after 3 hits (but true dmg), and kog'maw's is 6% per hit, but has a cooldown.
Add in his passive, and I'm not sure how his steroid is "vastly inferior" to anyone's.
Kog Maw - Near 100% upkeep with enough cdr, 6% on every hit instantly. Scales insanely with attackspeed.
He is weak compared to other carries.