[thejeff]

Sure, PD's never going to stand up to a determined attack. Big battlefield spells aren't in question.
Even Harbingers are pretty high-end thugs. Close to SCs if geared right. As they should be. They're a 6th level summon that costs 25 gems + gear. Though I've lost them to regular troops. If the mistform goes, they're dead. With enough troops someone will land that lucky shot.

But I'm really talking about the cheap thugs. Black Servants, Sleepers, Banes. Black Knights or similar mounted commanders. Sacred mages who can teleport and cast a few buffs. All with not much more than a Brand and a shield.
Any of these will take low level PD with little trouble, but might have trouble with larger numbers. Even without specific magic counters.

But since PD gets dramatically more expensive and can still be beaten by a mid-level thug: Harbinger, Bane Lord, sacred thugs etc, there's little incentive to buy more than the bare minimum.

Maybe enough to stop random events, if you've got good PD units.