Also, I would say current unstable injector is closer to useless than niche.
It's close to autopick for frigates - most frigates will never outrange anybody in meaningful way, so it's better to stop trying and capitalize on high speed instead.
Same for any size SO loadouts, as long as you can afford OP cost.
I'd also use UI on player-piloted Medusa, or any early DE while I have no ITU yet. Probably larger phase ships too, if I ever used them.
I agree that it's mostly useless for non-SO cruisers and capitals.
You don't have to outrange something for range to be useful. Increased range means you take less damage on approach before you can start dealing damage. Range lets you play at safer distances and take less 'standoff' damage which I find to be very helpful for the AI. The AI is pretty terrible at utilizing speed in my experience. For my flagship, I've never felt like a SO ship needed more speed, I'd much rather have more guns and vents, and I pretty much always run out of OP before mounts/dissipation. My SO ships are almost always SO + hardened subsystems + good weapons and vents with no room to spare.
Also, I feel like in the early part of the game where frigates are somewhat relevant, they can still be somewhat competitive in range, even if they aren't explicitly outranging things. Maybe I'm not giving enough credit to UI, and maybe I'm biased by late game, but it just doesn't seem very useful to me.