The other thing you're forgetting is that the larva mechanism allows the Zerg player to build a lot of a unit at once. If you get five vikings on the field, he can get 10 corruptors quicker than you can get to ten vikings.
During the Viking stage of this plan, you're incredibly vulnerable to fungal growth on the vikings. 1 FG on a group of eight vikings could mean certain death as a few queens slowly pick them off.