Never underestimate a 6" Shrouded bubble. Venomthropes can really put a shooting army out by handing out 5+ minimum saves to cheap fodder. For an army like Tau, that means spending Markerlights to remove cover on targets in the open, when you really need those Markerlights to make every shot count.
Zoeys have almost always been a bit underwhelming, though, in my experience.
That's not to mention anything in cover, which can get a 2+/3+. On something like an Exocrine hiding in Area Terrain, that's nasty.
If you have Synapse well-covered, ask a player for a friendly game and proxy the Venoms with Zoeys before you throw cash down on them. The best way to find out if you want a unit, is to play it.