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In the case of the Tervigon,, I would say it's obviously a balance issue.
Having 1 more unit per turn of gaunts with ObSec isn't going to give poor Nids an insane advantage. I'd find it wonderful if my opponent was stocking up on Tervigons rather than Flyrants or Mawlocs or whatever else is scary.

But it would be insanely unfair for one unit of cheap Pink Horrors to spawn two units a turn for the whole game that all have ObSec. A CADful of Horrors all with the Locus of Creation...that'd just be crazy.
....excuse me while I go make a silly list.