I can comment on the early game - minor Vipers are ok, they re mobile but concentrated fire from 2-3 soldiers kills them. On the other hand 2-3 Gargols will wipe the ground with unprepared player on the first mission. I think the problem is their mobility, they can cover too much ground, making them just like a tougher version of minor Vipers. I think you can balance them by reducing their mobility, so they will provide a nice alternative to the Vipers.
For now options against them are quite limited, best way is to reduce their cover using rockets and have 4-5 soldiers concentrate fire on them. I consider them a bit over the top for the early game.
A question: When new mods weapons will get updated tooltips in soldier equip screen? Its hard to see how Zephyr is better than basic ballistic rifle.