If there are sets of weapons that are unaffected/largely unaffected, then UI becomes an "always-best" choice for builds using those, because of how important speed is.
That's kind of the point; it's a buff to shorter-ranged weapons like the Ion Pulser that right now are very niche (essentially restricted to SO or phase ships), and it penalizes longer-ranged weapons
more; the Gauss Cannon gets cut down to 900 range instead of 1020, for example.
I agree that it becomes always-best... but I'm not sure that's inherently bad, because of which ships are affected. Capitals and cruisers already depend on longer ranged weapons; this makes shorter-ranged builds and hulls more viable. Ships armed with energy weapons already have few to no reasons to use Unstable Injector, because of the range penalty on top of the inherent range disadvantages of projectile energy weapons; this levels that out.
If the net result is that ITU/DTC are already must-haves for slower/ballistic-focused ships,
but UI becomes obligatory for faster/energy-focused ships (and maybe most importantly, offers a clearer choice WRT midline ships that can mount both; take ITU, kite-and-snipe, or take UI, become a short-range sledgehammer exploiting the combo potential of ballistic and energy slots), that could level the playing field in a couple of areas where we keep hearing complaints; frigates, high-tech ships, non-beam energy weapons. Right now, ITU is already cheap enough in OP terms that it's mandatory (IMO) on non-SO frigates; it's a little different when the choice is between 'which hullmod should be mandatory' as opposed to which hullmod should you choose between (and if your concern is that ITU + UI would become mandatory; make them mutually exclusive, force the player to pick one or the other.)
I'd argue that with weapons like the IR Pulse Laser, if you're relying on them you actually lose less range on an SO ship (500>462.5) than on a ship with Unstable Injector (425), so there's no real reason to use UI on those ships at all. It's even more pronounced when you consider short-ranged PD weapons that aren't affected by SO at all, but are by UI; Vulcan Cannons, LMGs, PD Lasers.
I'm coming at this from the angle of having designed a fast midline faction with lots of hybrid slots and wanting energy weapons to be competitive on them; perhaps that's not applicable to vanilla ships, but I think it solves a lot of edge cases and feels cleaner than the current percentage approach.
And 600 range also includes a large number of low-end small ballistics.
Yeah, that's a bigger issue. I dunno though - on something like a Lasher, is that really a problem? I guess it might be a minor buff to larger ships that use small ballistics, but given that those ships have short ranges as it is, UI would give them an option besides ITU, SO, or simply being second-best.