@ Null Ganymede:
I do not know what point when critical mass is reached. In older versions, Pilums spam wrecked fleets. Today, not so much, at least not without lots of missile spec officers. I do not know if officers will get the job done, but without them, I could not get the critical mass of bygone releases.
Just did a quick Pilum test, with three Dominators and five Falcon (P). Things I noticed...
Fleet as a whole needs to be built for Pilum spam.
At first, there are indeed clouds of Pilums, as all ships fire missiles, but it does not last for long. There may be too many to shoot down, but it does not matter if pilums are slow enough that enemy outruns them or blocks them.
Against a roughly equal sized enemy force or bigger, Pilum spam is mostly ineffective. It works better against fleets smaller than player's.
Without ECCM, Pilums are simply too slow. There may be too many to shoot down all of them, but the enemy has no trouble shooting some of them down and blocking or simply running away from the rest (they are already cowardly enough, don't want them to run away even more!) Pilums hardly scored any kills against enemy fleet. They did overwhelm singular targets like a lone battleship (but at that point, bring in lone Harbinger and torpedo it). ECCM helps a bit.