Alright, then I agree. There are points of articulation, like the armpit, that could be targeted with a knife. It wouldn't be easy, as you keep claiming, but it is indeed possible. If you stab them in the armpit, it could even make them drop their weapon... But it still wouldn't kill them outright. I mean, not unless you had a magic knife that could bend around to target the middle of a torso while stabbing up into the armpit. You'd still need to unseal and unlock the helmet, take it off, and THEN kill them before they managed to raise an alarm... Which, in 99 combat situations out of 100, means you're dead, because of a handy thing called in-suit radio.

But, anyway, with the right tactics and enough luck, sure. I'm not saying they're absolutely unbeatable... But they're pretty fucking unbeatable.

You'd need to be part of a much larger operation, with enough cannon fodder to distract the guards while you sneak up on them, and by that point, is it really worth it? Even though in a hypothetical situation a [i]single[/i] guard in power armour could be killed by a very lucky resistance fighter with a knife, it would require the sacrifice of so many other resources that it's unwise at best and suicidal at worst. We need to be smarter than that, if were going to beat these motherfuckers.

EDIT: Basically, we shouldn't see engaging the forces of the Monarchy armed with knives as a viable long-term option.