Ok this is unscientific but for the last two weeks I've been testing on Pineapple looking for my new favourite build. Tried various combinations of Brutes and Scrappers with:
Primaries: Radiation Melee, Electric Melee, Psionic Melee
Secondaries: Willpower, Shield Defense, Invulnerability, Super Reflexes
All were soft-capped to at least S/L, with SR and SD soft-capped to the three positions. Took them solo into a +0/x8 ITF to fight a bunch of Cimeroan traitors to see how they stood up.
Willpower is like an energizer bunny, just keeps going with endless health and regen until incoming damage overwhelmed it, then defeat.
Basically the same with Invulnerability, but with Dull Pain it lasted longer, took longer to whittle down.
Shield Defense hardly ever got hit but if health got towards 50% defeat was close behind. Sometimes instant defeat in the middle of a spawn.
Super Reflexes was surprisingly hardy. Defense-debuff-resistance meant my defense never went negative, whereas it did for the other three. I opened the Combat Attributes and as health went down Damage Resistance went up, to 50% at about 1/4 health. But the enemies could still hardly ever hit me vs guaranteed death by now on the others. Worst debuff was the Minotaur's Fetid Howl which did something like ~20% reduction to damage resistance.
So SR is Positional Defense +scaling DR +DDR +recharge +slow resist and counts as multi-faceted IMHO.
Shield cracked and died quicker than SR. If you have a self-heal from somewhere it's even better, like with Dark Melee. I tried adding the Call of the Sandman Heal Self proc to the first two Electric Melee punches but they didn't help much. 33% chance for a 67HP heal is a drop in the bucket. Haven't tried Aid Self yet though, may do a respec and see how it works while surrounded.