I wouldn't say so, most decks can win with just 7 creatures. You rarely need more than that to win. Only FFQ and skeleton decks need more than 7 creatures to deal some serious damage (even
spam can win with 7 creatures if played correctly).
Of course if opponent plays flood+aflatoxin you might be limited to less than 7 creatures, but it's your problem to prevent this (have some creature control that can target your creatures, or play only immortal creatures, or do not play creatures if you see a
deck, just as you do not play low HP creatures if you see a
deck and expect otys).
And if you play rainbow, try mutating skeletons/fireflys until you get a water or immortal one ;P
EDIT:
As a second thought, hope and fractal decks might also have problems against flood, but against any good deck/strategy there are decks that have little chance against it, as well as those that deal with it easily, so I don't see it as a sign of flood being OP. In fact fractal and hope are still really good cards even if flood counters them in most cases.
And I guess scarabs (both fractal and pharaoh ones) can deal with flood, because 7 scarabs or 6+pharaoh is good enough, and they can eat any surplus scarabs