With a brood of 3 zoanthropes in the past, you could choose to use warp blast or not on an individual basis, and you would have a far better chance of getting, for instance, FNP. The access to a second power is pretty much useless. The odds of actually getting warp blast are no better; in a sense they're worse. Now it's three or nothing, if you have a full brood, in the past your risks were spread so you had a better chance of getting a single blast. In addition, previously you would have had access to three second powers, now we have one. We're paying 5/6 of the points for a unit that is 2/3 as useful, or less.
I don't think you know how Zoanthropes worked before. You didn't have a second power you had Warp Blast or a book power so you were stuck with one or the other for the entire game. So they now have more utility. If you kept Warp blast on them before you could shoot with all or none you could not shoot with some and cast different powers with others one turn and then shoot with all another turn as you contest. Now I can hide 2 out of
LoS and still fire 3 shots at the enemy. I can choose which model all 3 shots come from so that helps me get more side armour and ignored more cover.
So they gained utility by having a second power. They gained the ability to fire all shots from 1 zoanthrope. They gained the increased importance of Synapse. They gained comparatively with their main adversaries for the
AT shooty Elite role. Whilst dropping 10 points. They are better for less points.