I hope this doesn't take things too OT, but this is sort of a pet-peeve for me. I think that GW royally messed up with having the following saves: armor, invulnerable, cover, none. That's it. Then, all of a sudden we have weapons that ignore invulnerables - not just invulnerables, but all 'invulnerables'.
The easier fix is to pick 3 or 4 types of invulnerable saves, and codify them: foe.g.: field, daemonic, rune, dodge, cover. Then, for any unit or model that has an invulnerable, it is listed as one of the above. Then you can make a weapon like the Psycannon that ignores 'daemonic' saves. The rest are obviously still legit.
I mean, seriously, we could all handle a wee little bit of complexity to the rules, and really, how much harder is this to do??
Sorry for the rant, it just seems that we keep trying to patch the existing rules that have a fundamental flaw a little deeper.
I keep saying that with a handful of people from this board, and about a week, we could probably edit the 40k rules, fix 99.7% of the langauge problems, and we'd have a stellar ruleset...and I'd do it for free