The Brawler is just terrible, but there is one other ship that just outright irritates me: the Sunder-class. Not so much because it's bad, but because it feels as though it should be so much better. The large energy mount seems like something that would be genuinely valuable, but the complete lack of any fighter defence leaves it so vulnerable that it just can't be relied upon to stay alive even, with an escort.
As for the Conquest, though, in my opinion there's really only one problem with the ship: the default loadout is truly terrible, for two reasons. The first is that, as people have mentioned, terrible flux capacity and Mjolnir cannons do not go well together. In fact, terrible flux capacity and capital ships just plain don't go well together. The second is that it basically ignores fighter defence: really, those lasers aren't good enough to stop Pirahnas without moving your shields to deal with the threat - leaving you exposed elsewhere. My solution, therefore, is first to lose the Pilums entirely: as long-range artillery, the Conquest just doesn't have enough launchers to prevent its missiles being shot down by anything that's slow enough for them to catch. Replace the Railguns with Dual Flak Cannons (make sure to set them to another weapon group), which deal with bombers and missiles nicely; you really don't need the railguns' added firepower to back up an already impressive broadside. Finally, replace the Cyclone launchers with Typhoons - really, who needs *20* torpedoes more than a total of 32 extra OP? All this allows you to shove in capacitors and vents until you can support the Mjolnirs; or add in Gauss Cannons for their flux efficiency so you can add in a whole bunch of hullmods.
If I'm honest, though, even then it's still just not as good as an Elite Onslaught. But the much nicer aesthetics of the Conquest are usually enough to make me pick it instead.