[quote=Imperfectionist]
There aren't any "openings" at all, the suit and helmet together are against the outside world (which is why the armor grants a resistance to radiation). The helmet locks into the armor, and the neck is protected. In fact, this Royal armour is actually something of an odd duck compared to the major armors over in America (meaning the T51-b and Enclave Advanced armors), as those lock in so seamlessly there's nothing but an impenetrable crease. And, stabbed in the neck =/= severed jugular or instant death, especially with the protection that I've just explained.Look, all I'm trying to say here is, if Adrian attempts to use a knife to kill a fully armored Knight, he's not going to succeed. He's going to need better tactics than that.
[/quote]

I had a closer look at the armor and you may be right. The neck is probably protected but not EVERY single piece of the body is covered by the suit. There are a couple weak points that a knife can easily penetrate. The armpits and the small opening by the groin.

Yes they do have a large resistance to radiation but that doesn't mean they are fully protected from it. Every single piece of armor has a weakness. 

The armpits would be probably be done from behind in a sneak attack and the groin would probably be a lucky shot. 

My point is, a knife can kill a power armor guard. You don't need a massive weapon to do so.