Thanks for the write-up Ruck. I have a couple thoughts about the FSA's B-72, as it is the only one of the heavy bombers with which I have experience.
Your point about the Heavy Bombers having to distinguish themselves in an already-crowded field of medium flyers (at least for the core seven nations) is well-taken. My theory, though, is that Spartan is giving the core seven nations a wide variety of models for a wide variety of situations. (I see the alliance and merc factions more as "specialists", but that's my own theory.) The B-72 is a good example. For medium-class flyers, the FSA can field the PYB-2, the Lee scoutship, the John Henry robot, and the A-17 torpedo bomber. However, the B-72 is the only one of the FSA's medium flyers that has Piercing Munitions. This gives me a weapon that can help me deal with Ablative Armor or shields--I still have a chance of causing critical hits without having to get past CR ratings. So the B-72 might be more useful when I'm facing the Russians or the CoA.
Also, regarding the Low-Level Flyer ability: this gives the B-72 more survivability on the approach to the target. It's a medium capital model, and once it goes low-level, it is considered to occupy the surface height level and becomes essentially a surface skimmer. While it also gains the "Vulnerability" MAR and can't use bombs, it also means the B-72 can mask its approach with both surface ships and terrain.