I've never had an issue with seeing them land myself; they're pretty obvious. Even more so in his purple skin (the blobs have little tentacle that whip around), but even in base skin I've never had a problem seeing them.
And actually he's one of the few Champions whose health mechanics actually work. That is, that they have the same kind of cost/reward system that Mana or Energy has. If he can get his blob, he gains health, because it costs 4% of his current health but returns on his max health. So it is always a net gain. Vlad's works because of his scaling mechanics and the health returns on his Q (amplified with E) but Mordekaiser, for example, requires spell vamp so that he can recoup some of the costs of his spells.
Mundo's works post-6 with the massive regen on his ult (which is better now than it used to be without Culling and the dwindling number of Grevious Wounds sources) but laning phase he still has a really hard time.
Edit: The fact that Zac's blobs move somewhere between him and a nearby enemy Champion is the only way to give the mechanic counter play. Before that, Zac could be ran top lane because he had stupid amounts of sustain and could out-sustain Vlad and Renekton easily.