Skirmishers feel fairly balanced to me.
The really strong ones like recluse cost a fair bit and that gives you a while to react before they become OP. Rogue I feel isn't "really" a skirmisher but more of an artillery unit in shields. Because I find shields are best used in bursts (attack, retreat, recharge, attack again) I load up on Rogues to have a punch vs static defences.
But anything in huge numbers can feel overwhelming, I guess.