Useful in that they deal damage, but not practical.
I incidentally loaded up my gunship with 4 flak launchers, a C3 beam and two C1 fb rails to see how it went. Unfortunately especially without engineering, they just don't outperform anything in any way. As an intrinsically low fire rate fixed projectile based weapon they cannot be thrown effectively at long range, but even at closer ranges when able to get all 4 launchers to explode in close proximity, the damage wasn't great; flak relies on pumping as much of its payload into the target as possible, so if the flak launches its payload in literally all directions, at least half that payload is being lost.
I got a few good shots off - took a few chunks out of Asps, T9s and the like that have a good surface area (fun fact: the flak launchers literally don't detonate until release, and will bounce off an enemy's hull), but ultimately nothing I couldn't have massively outperformed with a standard flak launcher at close range or at least equalled multicannons or missiles or whatever at mid range.
