Ghostwalker has some problems. Probably the biggest one is the fact that it's a very "solo" archetype - for example, the shadow walk ability is likely to leave the rest of the party behind. And how often will the character be seriously wounded in an encounter, survive, AND confront the same foes a second time? IME parties of PCs tend to either wipe out the opposition or be TPKed in turn. Finally, trying to turn The Man With No Name into a PrC is like trying to make a Batman or Wolverine PrC - the original character is both unique and powerful (high level) so either the PrC must be weaker than the original (and hence disappointing) or quite powerful (and hence probably "broken"). Plus the resulting PrC tends to be very narrowly focused, often too much so IMO.