Onslaught is the weakest Capital. But I say that of lots of capitals, so take that with a pinch of salt. The Onslaught does one thing extremely well, and that's to mount a lot of flak cannons. A pair of Squalls? No problem. A mass of piranha bombs? Who needs to dodge? Not the Onslaught, because it cannot. The paragon can't dodge either, though I guess the Paragon can just turn fortress shield on. I guess the Onslaught fulfills the role of an interesting ship to fight against. Frontal burst damage, tough against low damage weapons, weak rear.
Playing with a fleet of them, compared to Conquests, they are much more likely to get into trouble and not be able to withdraw behind another ship in time, but conversely, they are much more able to survive long enough for help to arrive. Compared to Paragon, they are terrible, but can chase down withdrawing ships, even destroyers, that the Paragon cannot kill in time. Compared with Odyssey, they got longer range and so can ward away frigates and destroyers much more easily. Then again odyssey can chase and retreat and just kill them. I suppose the best you can say of the Onslaught is that it is a good place for your other ships to retreat behind. That's not really high praise though.