I feel like SBD is weaker at long range on account of its slower move speed making it much easier to hit than other classes at the same range, buuuut that's just a feel, may not hold up under examination. At short range as an SBD I have a much greater feeling of "sure, you can hit me, but you can't stop me from hitting you either," while at long range I feel like I just get pecked down. I'm certainly "playing it wrong" to a degree.
But certainly I would argue that a P3 Hero at long range has a huge advantage on an SBD. Hero gets to run and fire strong single-shots while SBD has to walk slowly to fire accurately and individual hits do less damage. Mag plating maybe makes it a bit more even but then you're either more fragile overall, have less firepower, or are extremely vulnerable to sabers.
I feel like a more accurate characterization of SBD is that it can be configured to be strong at any range, against any class, but it is never the case that a single SBD is strong at all ranges against all classes. And there's always pulse nade ARC
SBD's totally the best single anti-Jedi gunner on blue, but in larger group situations, flamethrower and poison and stuff can be more useful than the SBD's sheer imperviousness. And when it comes to gunner vs gunner, I think I'm more scared of a good player playing BH than I am of that same player playing SBD? But then again, maybe I don't see many good players playing SBD.