First off, the deck is fast. Very fast.
Flash Hulk can consistently happen turn 2 or 3, and turn 1 isn't uncommon. Because it's so early, this is a rare case where someone might not have an answer. Getting one is still possible, but it forces you to change your
game plan. For example, my ideal
game plan with Edric is to spend the first two turns playing low-cost evasive creatures, and then start the draw engine on turn 3. When I'm up against
Flash Hulk, I have to change that plan so that I always have 1 blue mana open. Even if I don't have a
counterspell, I need to
threaten that I do. This unfortunately slows me down by a turn or two.
Second, the
Flash Hulk deck doesn't have to work around a massive hurdle if it gets stopped. If
Helm of the Host gets destroyed in a
Godo, Bandit Warlord deck, the Godo player is either out of the game, or has to get recursion to get the helm back. This takes time. There are other combos that the
Flash Hulk deck can fall back on, quite easily.
Third, you need to understand how the combo protection works. The most common version I've seen is where they cast
Flash, sacrificing
Protean Hulk, and get
Cephalid Illusionist,
Nomads En-Kor,
Hapless Researcher, and
Grand Abolisher. That last creature locks out any interaction the opponents might have, so the game is already over at this point. Because of this, you have to respond while
Flash is on the stack (removal is useless, it has to be a
counterspell or
Containment Priest). You also have to remember that the
Force of Will and
Pact of Negation that can stop the combo can also be used to protect the combo.