I promise you, Wiki, unless you've got a Protonic Axe or a Ripper or something, you aren't going to be able to slit a power armored soldier's throat. It just ain't happening (as long as they're wearing the helmet).
If an obvious target like the throat wasn't protected at least enough to repel knives, the armor wouldn't have made it past testing. In a nutshell, it's designed to protect the wearer in
every possible combat situation, including close combat, meaning knives =/= instant death. Not even close. In fact, looking at the image Sep used for the Royal Knight's
power armour, the shoulders and back of the armor are designed such that the only way to target the soldier's neck with a melee weapon is to come at them head-on AND at an awkward angle (i.e. you die)...
There's a reason why it's so impressive, and so feared on the battlefield, you know. The best method to kill them, in my opinion, is to trick them into removing their helmets first, as otherwise there's pretty much no hope (except
maybe C4 or other very powerful explosives). Read the story of
Maxson II (the second leader of the Brotherhood of Steel) for a good example.