Energy torpedoes have -50% vs shield, not vs armor. Against armor they get a +50% bonus. Mixing them with plasma creates ships that get hard-countered by shields, but melt armor. That said, if you have other ships to deal with shields (say battleships with gigacannon + kinetic artillery + afterburners) they can work very well.
However, energy torpedoes are bad vs the small ships the AI likes. Their base damage is too low. Against cruisers they are average and against battleships they are great, but you don't want to use them against destroyers let alone corvettes and frigates. Regular torpedoes on the other hand will already do good damage already vs corvettes, great damage vs destroyers, awesome damage vs cruisers and insane damage vs anything bigger. Their flip-side is low range and that PD can shoot them down, but AI will rarely have enough PD to do so and short range isn't as much of an issue if the ship carrying the torpedoes is tough, like say a cruiser. But if the torpedo ship is fragile, like say a frigate, then the short range will be an issue.
That's why I equip my cruisers with regular torpedoes (and disruptors to finish the job if needed), while my frigates get to carry energy torpedoes and regular missiles, to keep them away from the frontline.
But to get back to the original topic, I think small ships being fragile is pretty much working as intended. What isn't working as intended is the AI ship designer. This is because some time ago someone at PDX came up with the idea that the AI should use how much power a weapon uses as a proxy for how good the weapon is. And I think that was actually a pretty clever idea, it's just that the combat rebalance didn't take it into account properly and now the AI thinks that plasma and autocannons (which use abnormally high amounts of power) are high tech superweapons while swarmer missiles (which use very little power) are low tech trash. It's so bad that even AI empires that are supposed to favor explosive weapons will still rather use a plasma & autocannon combo instead of using swarmers. And it's the main reason why all late game AI designs look so similar.
What that has to do with corvettes dying is that autocannons are the premier anti-evasion weapon. Throw in advanced sensors and their tracking goes all the way up to 100%. And AI absolutely loves those damn things. Just about the only empires that don't use them en masse are those that favor energy weapons (which will go crazy for plasma instead). AI also likes to use more small ships than the typical human player (or at least the typical human player who visits the forums) and those ships mostly won't have large weapon slots to put in weapons that corvettes should be good against.
Long story short, the broad combat system is fine, but AI ship design and fleet composition is largely optimized for fighting small ships. That is why you're experiencing the problem you are experiencing. In the short term your only two options in singleplayer are phasing out the use of small ships or combat behavior mods. As for what PDX could do to alleviate the issue in a future patch, the simplest and least disruptive solution is to standardize weapon power usage, so that all weapons of the same size and tech tier use the same amount of power. That would diversify AI ship designs away from the autocannon spam, which I can confirm would work because I tested it out.
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