I've been playing with DEMs lately, in particular using Gorgons on Enforcers (in a linked group of 4, not tied to weapons) in the early game, then more gorgons on Dominators (a mixed low tech + DEM playthrough) in the midgame. When fielded in decent quantities, I actually find the opposite of this:
they are also entirely useless vs majority of high tech
Versus armor based cruisers, which often have moderate PD, they are not nearly as effective as harpoons with how the AI uses them. They get through the high flux shields easily, but at the same time they are short enough range that range-boosted flak can shoot half of them down, and their anti-armor and anti-hull damage is much weaker than a harpoon. They hit, they do damage, but they just aren't as effective because the part they are good at (anti-shield) is not needed.
But versus high tech enemies they are doing better. Yes, said enemies have good shields, but Gorgons have much better (slightly better than 2x) anti-shield performance than harpoons, and the high tech ships often have less capable PD and less armor (though not none on the big ones). The AI only fires when the enemy ship is high on flux, so I've been finding that the Gorgons can punch through the remaining shields and then do damage in situations where harpoons wouldn't.
I will say that in all of this testing I'm supporting the Gorgons with long ranged kinetic damage. They're being used as supported finishers, not the sole weapon trying to take down shields.
One thing though: The small Gorgons are good in relation to small Harpoons: more ammo! But the medium Gorgons are really crap compared to medium Harpoons: they only fire 2 instead of 4 and have the same ammo, which for a finisher weapon is a crippling disadvantage.