Initially I thought the OP was referring to the Paragon, and I was a bit confused, as that Cap still has pretty solid armor despite being an obvious (and slow) shield tank.
Seeing Sunders & Colossus IIs called out specifically, I think there's a lack of understanding on them fundamentally. Neither of them are "shield tanks" and in fact have pretty poor shields for their type: Sunders have one of the worst shield efficiencies in the game at 1.2 flux/damage, and the Colossus II just has too poor flux stats (both dissipation and capacity) to be tanking anything. Neither of these rely heavily on shielding to survive (not like a shield tank does, at least), rather they both use heavy firepower (and in Sunder's case, range) to kill or at least ward off other things before they die. Since Sunders have, or can acquire, a considerable range advantage over most other destroyers (Beam loadout), they can accomplish this more easily than Colossus IIs. Of course, a Sunder can't surprise you with 20 Hammers to the rear arc, either.