Strong evolutionary DIS-advantage compared to non-fixing plants in any non-nitrogen-deficient soil. Doesn't matter if you fix nitrogen in a chemical plant or a plant root, it takes lots and lots of energy.
Its a natural ecological cycle like many others with oscillations and such. Given a couple decades, a soil saturated with nitrogen would probably result in the non-fixers wiping out the fixers.
The more flippant answer is it can't be any worse than Kudzu. Kudzu is an edible (edible by goat standards) nitro-fixer that is eating the south. I would guess that issuing a human edible competitor to Kudzu would be a net win? Cover the American North with kudzu-like layer of super-wheat wouldn't be so bad...