Phorids do have a major drawback -- they are not terribly host-specific. This means they will attack native ants, too.

The argument goes like this: Fire ants attack and extirpate native ants. Therefore, the fire ant population far outnumbers the native ant population. So introducing an ant parasite will differentially impact the fire ant population. Furthermore, phorids do not prey until extinction. So they wont completely annihilate the fire ants or any other species or population. They will attenuate the density of all ants.

Will it work?

Phorids have been introduced across the Gulf Coast states with several viable populations established. In those areas the fire ants have been reduced but not eliminated. Long term? It's an experiment. We'll all find out.