Safety overrides are their own thing that I've not much experience with. But I'm going to assume there is a damn good reason why you can't put it on capitals in the base game. SO Hammerheads are also have legendary status for their devastating effect, so if anything SO makes anything 'better'...... for limited periods of time. It used to be similar with Unstable Injector as you could just pick your engagements faster then any enemy could reliably react. High tech does this by default but I've never considered that to be too much of an advantage in the kinds of "line battle" that I usually play.
And Apogee's are fine, better then Aurora's for me but honestly it's the starship enterprise of the setting. The best out on the rim but more of a shield tank that gets replaced by a proper capital later into the campaign. Not my choice for a line ship that needs to be dealing damage as well as taking it, not that it can't but against other hightech shields one large energy of any type just isn't cutting it. Which is the main issue with high tech and energy weapons. Shields.
It's VERY easy to conclude that low tech is poor (or high tech is great) because you sent most of a game tachyon lancing pirate clackers with terrible shields, only to fight a remnant ordo fleet and find your shots harmlessly bouncing off their shields for basically no effect. That's when you need something decent to deal that damage for you (and who can't equip sabots?)
I have successfully played a (mostly vanilla) Luddic Path game with basically just overridden ships and a pair of capitals already coming with unstable injector (Tyrant, LP variant from Vayra if you're interested). You would be amazed at how blazing fast you can end battles when most of your force has safety overrides, Sabots, assault chainguns and hammer barrages, especially high tech and [Redacted] since they usually over-rely on shields and once that gets saboed to hell and back they have very little options.
Aside from the obvious drawbacks of reduced range and PPT, Safety overrides can make a ship useless unless you carefully fiddle with it. Reducing it's weapon strike capabilities to point blank ranges can result in the ship never being able to properly engage if the enemy deals enough hard flux damage to force even a reckless commander to backpedal.
Hence why high tech ships getting overridden is a hidden gem barely anyone considers. They have the best shield damage ratios and flux capacity/dissipation in the game, making them handle safety overrides a lot better and even warranting the use of hardened subsystems due to just how lethal they are even just by using hard flux energy weapons and not missiles.
The Apogee is a great example. It lacks speed and has great long range capabilities but turns into an unholy hammer of destruction with safety overrides and a Plasma Cannon. That shield on the Apogee is so damn good only things like gauss cannon wielding skirmishing ships will be safe from being run down and turned into swiss cheese.
Important role in fleet progression? Since when? I'd take an eagle or falcon over it any day. Besides, just saying it has "capital level firepower" doesn't really tell the whole story when it still has weak flux stats meaning it can't sustain that firepower if it actually gets into trouble. When it does get into trouble it has no escape and eats a few reapers or some more hammers and goes down.
It does have its strengths but from my decent amount of time playing I have rarely seen them ever be used to good effect.
I will agree with you on the fact Dominators usually suck ass when they're the opposition (mostly because the AI sets them up in a rather mediocre manner) but they are far, far from bad in a player fleet.
Calling it an early Onslaught is diminutive, Dominator brings a whole lot of health, armor and staying power to the field and you can do a really vast array of things with it.
You can give it PDAI, double flaks on the medium slots and flux efficient large ballistics to be an effective frontline element getting attention and dealing a lot of hard flux damage
You can override it, give it aux thrusters and QUADRUPLE ASSAULT CHAINGUN with 3 Sabot MRMs and turn it into the bane of anything smaller and/or slower than itself.
You can give it double gauss and pilums for that sweet long range capabilities, low flux dissipation won't matter if the enemy can't get close enough to exploit it.
In all cases, you get a ship with near capital levels of armor and HP that will just keep fighting and just not die unless something goes very wrong in the battle, wich tells a lot more about a fleet's inability to exploit enemy aggression than the Dominator not being good.