Advanced Targeting Core, ballistics and Plasma Jets all contribute to making Guardian the best conventional soloing ship in the game. Capitals, as well as low-tech and midline cruisers are too slow to escape the mob, while destroyers, frigates and high-tech ships are too short ranged to attack without putting themselves in mortal risk. Only Guardian gets the winning combination, and talking about Guardian's strengths without mentioning this combo is incomplete.
Soloing is like an equation where defences, offensive power, mobility and range all play part, but mobility contributes more, the more range you have, and vice versa. You could substitute all other qualities with just having a ton of one, of course, but no ship in vanilla (and probably modiverse as well) has enough defence to outlast everything, enough offence one-shot-kill anything or enough range to never be attacked, but there are ships mobile enough to solo pretty well (in fact, it's pretty much the god stat) thanks to skills, like Doom, and Guardian has a balance of these factors to get a score high enough, to outgun what it can't outrun and outrun what it can't outgun, to be able to solo an ordo, which is pretty good, and it makes it the best conventional ship in the game for this purpose. It's "just" the best kiting ship in the game (honestly soloing is just kiting with no back up, don't try it), something that can match Doom or Ziggurat in some aspects (some, as I don't expect Guardian to fare well against doritos - targets are too small and too erratic moving, and I want to avoid missiles for these tests so Alex doesn't go "hmm yes, guardian but no infinite missles for the player" and my tests get outdated).
Guardian also gets unlimited missiles, which gives it a lot of offensive power, but other ships can imitate it and it doesn't let it do anything unique. I wonder if Guardian can actually solo a star fortress - it's something I assumed, but never checked...