Of non-high tech frigates, I actually think centurions are in a good (read: powerful) place. Not quite as good as omens and slower, but nearly so, especially with SO (there's a reason they are favorite tournament power ships). If not for the officer density issues that Hiruma Kai raised, I'd say that Centurions at 4DP are significantly more powerful than Tempests at 8DP.
I haven't played with Brawlers enough to know if they are really worth 6DP, but I used a few of them and they were quite effective escorts due to the range. Vigilances are weird in that they are terrible early game, but ok in mid game fleets that care about missile/DP density (though at that point just take a few Gryphons). They could stand to be cheaper DP wise to enhance the missile spam strength... hell, give them autoforge instead of fast racks and make them proper mini-gryphons. Monitor is a meme its so effective, Shepherds are excellent exploration ships and early game escorts (and turn into decent freighters with a hullmod), though not really true combat ships.
Anything without a shield I don't count, as those are not real combat ships.
Really its only the Lasher that stands out to me as being meant to be a combat ships but a step below other ships. It has enough flux to run an efficient set of ballistic guns (a mixture of railguns/light needlers, light mortars, and lags has significantly better firepower/flux than energy options for small scale fights), a firepower booster, good missiles for a frigate, and excellent point defense... its just really slow for having such weak defenses. Its the same speed as the Centurion, but has an entire ship class weaker defenses. Its closer in speed to the Brawler, but Brawlers are actually faster because of the system while having better range, shields, armor, hull, and flux.
If we want to go crazy and possibly overbuff: boost the lasher all the way up to 180 speed. It would still be a glass cannon, but suddenly it could swoop in and out of combat and would make for a nasty torpedo ship. With a railgun/needler, they would be able to keep up with and kite an omen, wolf, or other energy frigate from out of range unless the enemy had beams. SO Lashers would (like SO energy ships) be terrors.