tbh, if you are going for filling enemy hands for cluttering, it only stops them for 1 go, stops them drawing for 1 go, then they can continue as normal, as you can't draw the 1st go after it is played as you already have 8 cards, after that it is discard 1 you don't want and then you can draw again (unless another nightmare is played) so effectively it is a 1 turn stall
the only other benefit is it stops Hourglass reliant decks down as the player can still only really draw 1 card a go, and hourglass decks are usually rainbow so the cards from nightmare can be used eventually
the only real thing this card will do will force you to possibly play only certain cards, E.g. photon (which imo everyone would play if their hand was filled with 8 of them) and then the person nightmaring would just put up a shield to block damage for 1 point = 8 lost spaces on your side -- followed by repeating nightmare photons until you realise not to play them >_<
in short - this card - does nothing but delay for 1 turn's draw