This is a very good card, as you can make the opponent lose cards due to a full hand. This is especially the case when you place the Annoying Dog after playing the the River Person. It also works very well with Lemon Bread.
Countering the River Person is somewhat easy. Due to it's high cost, it might not be possible for the owner to place another card, especially before Turn 10. The opponent can then use Expulsion (in Integrity) or Endogeny (in general) to put the River Person back, making the owner lose the next drawn card instead.