While the Hargh'peng is the only torpedo broken in the direction of being overpowered, it's not the only one that is broken. It's kind of screwed up that canon ships almost always had a torpedo launcher or two, but for many ships in STO it's really more effective to forego torpedoes. Only Tricobalts seem to occupy thier proper niche.
The basic problem is that while energy weapons are designed to beat shields and torpedoes to beat bare hull, in PvP shields are almost never down completely. Even without buffs or healing, just distributing shield power is often enough to keep a sliver of shields up and keep torpedo damage inconsequential. Then consider the fact that every player has some form of shield healing, and won't allow their shields to stay down long enough to risk a torpedo hit.
I know that there are ways to land a torpedo. You might time the torpedo to land a split-second after a high burst like BOv, use a stun to keep the target from raising shields, or use drains and science powers to take shields completely offline. But using energy weapons to beat down shields and then using torpedoes against the hull is virtually impossible.
Transphasic and Plasma torpedoes should se more effective against shielded targets, but those torpedo types are just too weak to do their job. 600 point hull hits from a transphasic torpedo really aren't helping to bypass shields, since the damage is so low that HE alone will heal it before another torpedo can land. And then there's plasma, which just doesn't do enough damage from the DOT to be worthwhile.
Every torpedo type (except possibly Tricobalts) needs to do more damage. It could be in the form of a better multiplier against shields, better damage from the torpedoes special ability, or just more damage, but torpedoes are weak enough to make including them on many ships questionable.
I don't have a lot of hope for a change though, because NPC ships are idiots that don't do anything to raise dead shield facings, so torpedoes work great in PvE as they currently are.