I can't tell you what underperformed about the Mercury because it's a "feel" thing. Like I mentioned, it lacks killing power. It feels like it lacks "oomph." 6 small mounts and a mere 2 medium isn't much firepower, even if you put 4 assault guns on the front, 160x4 at 600 range just doesn't seem to cut it. I had success mounting a typhoon or an annihilator on the medium universal mount. But, compare it to the Eagle, or the Dominator, or any other similarily priced vanilla ship, and you'll see it's woefully undergunned.
Honestly, I like seeing the IF as a faction which doesn't rely on fighters for much. I like that there is no dedicated carrier, though we might be able to do with another flight-deck enabled destroyer or cruiser. Perhaps IF fighters will all be large corvettes who don't rely on flight decks too much. I know I'd like to see one with a rocket launcher of some sort, instead of bombs.
That said, the frigates still need work. Now that my fleet has flight decks I've completely phased out frigates for fighters. It's a late game fleet, sure, but I just don't see much use for them. The cargo capacity sure won't help me, and until the AI is made smarter about self-preservation even the Dakota's efficient shields can't save it from its own stupidity.
I just wish the station would receive more Toas and Defiants. Now those are competent vessels.