I used to think that the normal Legion was trash compared to having two large missile slots on the Legion XIV. But I have found I much prefer the normal Legion given how easy they are to find! In my current Nex playthrough I destroyed a Luddic Church vengeance fleet and was able to salvage a Legion with 3 s-mods. I don't even have the skill for that! It has Expanded Missile Racks, Reinforced Bulkheads, and Heavy Armor for free, thank you Nex!
The fact that you can put 5 Annihilator pods on the Legion is amazing. Paired with expanded missile racks and officer missile skills, you have 360 shots in each missile pod. The large ballistic turrets let you put in Gauss Cannons, one of my favorite weapons in the game with very long range, high projectile speed, and very high per-shot damage effective against both heavy armor and shield.
The Legion doesn't have great flux stats but maxed vents + flux distributor gives just enough flux to power 2 Gauss Cannons, and the 5 Annihilator pods are flux-free. Who needs to power a shield if you have 2000+ armor and stay at 1500+ distance? This setup annihilates stations below Star Fortress size, and can fully occupy the point defense of one entire side of a Star Fortress as part of a fleet of capitals. The Legion is also really hard to kill--heavy armor, reinforced bulkheads, and vulcans in all the small projectile slots make it a tough bird.
I think all the low-tech capital ships are hamstrung by bad flux and shield stats in the end. The best use I have found for my Legions or Onslaughts is baiting the fire of scary targets while my two Paragons actually do all the work. Legions are also very half-baked when it comes to being carriers. Unlike an Astral, you will not be deleting cruisers or larger with waves of teleporting bombers. If you spend the OP on good fighters, expanded deck crew, recovery shuttles, etc., you will gimp the rest of your Legion fit. I prefer Sparks personally, because 4 Spark squadrons are cheap on DP but fairly effective when massed against harassing frigates and destroyers. Xyphos would be interesting to go with a Battle Legion too but you might have to forgo hull mods or flux stats to accommodate them.