Used to be worse before the G&K expansion. Under the old 10-hit-point scale, even the lowliest warrior or archer inflicted a minimum of one point of damage, or 10%. Surround a GDR unit with 10 archers and watch it die in one turn. Silly, really....
And how many times will Archers be able to survive long enough to completely surround a GDR? How likely is it that a civilization is still producing Archers when GDRs are present?
Zone of control makes the prospect of surrounding any unit unlikely at best, unless the unit in question is garrisoning a city. Moreover, how likely is it going to be that a civilization in the Future era doesn't have ANY units to support a GDR against a swarm?
Moreover, Archers have been long out of production by the time GDRs roll around.
Pointing out how broken it is that Archers can THEORETICALLY swarm a GDR successfully is like saying that the Great Library is broken because it can THEORETICALLY be used to bulb some expensive Modern/Future era tech like Nanominiaturization. I mean, it could, but how likely is that?