I don't think it needs a nerf, the thing with Onslaught and Radiant is that it's so easy to build, just slap autofit on them and make a few hullmod tweaks and you are DONE.
Not all capitals are so easy and effective with autobuild and that makes them seem weaker.
Yeah the onslaught is really tough to build improperly due to TPCs at least giving a baseline of firepower even if everything else goes out of wack.
I know the favorite point of comparison is the Conquest, but the Executor(thanks) is an even more egregious example, especially since the Usurpers gives you one.
One of its autofits is legitimately designed to be terrible, and the Coherer and HEF point to putting high intensity med energies even if the ships actual design (midline capital) suits it better to beams.