Big dumb creatures are easily dealt with, most decks play some sort of unconditional removal, many play mass removal of some kind. Also, the Nighthowler is an enchantment, which makes it even more vulnerable. Graveyard hate is a thing, and deathtouch, and forced sacrifice, etc. Plus, it can be countered when you cast it. Aura based removal ala Pacifism deals with it entirely. In short, no, a big creature doesn't mean you win. Nighthowler is a bit harder to deal with than say a Gigantosaurus, but it is not at all gamebreaking.
Former Rules Advisor
"Everything's better with pirates." - Lodge
(The Gamers: Dorkness Rising)
"Any sufficiently analyzed magic is indistinguishable from science."
(Girl Genius - Fairy Tale Theater Break - Cinderella, end of volume 8)