>As a necromancer, you should always feel like you can beat
>anyone,

true

>but at the same time, if you slip up, anyone can beat
>you.

Uhm, no
You just retreat and return with better plans.
As a necro you don't have to fight and make a decision how
it is going in the middle of combat.
Cast sleep, failed? Alright, retreat and try again later.
Add the fact they have summon ( and thus don't have to actually charge into groups )
and i would consider necromancers as one of "no-risk-at-all"
classes in the game along with healers and assassins.

As for their good pk ratio,
That is their lethal power combined with the fact they don't
have to risk to land a kill.
Also most of sceptres are skilled players, many of them pretend
to be liches and newbies just don't bother to roll/rank a sceptre.