Yeah, it seems like they are only good on mobs from the previous biomes instead of being useful on the ones you meet up with after getting the materials to make ooze bombs. They appear to need either a slow factor as you suggest or a longer range, larger AOE, and both a higher per tick damage and faster tick rate.

I haven't yet run headlong into a situation where I faced more than 4 fulings and I did try the ooze bombs on them. They spent a lot of time running around outside of the cloud instead of inside it and none of them lost more than 1/10th of their total health before the cloud was gone. For the 0 star ones a block and 2 hits was faster.