Without getting into bloat issues of Monero, or Darkcoin instamine, a PoW coin, if actually able to achieve anonymous transactions, is in a difficult position.  Governments would regulate against it, and PoW mining data centers are large attack vectors to be legislated against or just taken over.  Yea, they would be located in many countries, but governments seem to cooperate a lot on things like drugs, so they might cooperate on issues like this too.  I guess you could say Bitcoin sort of faces this issue as well, but they haven't made any big moves against it yet.