In a 1v1 straight up manfight, the Victory is nearly unbeatable. It has incredible forward firepower; 2 larges, 6 mediums, and 2 built-in heavy burst DPS weapons. That's its job, that's its name; it is victorious. The Victory falters when it can't do that. Unlike the Onslaught, the Victory's anti-flanking weaponry is also its forward weaponry; the Victory has to split its firepower to deal with the flanker, where the Onslaught has dedicated side weaponry independent of its forward battery. The Victory has a terrible shield arc, only capable of defending against threats directly in front of it; the Onslaught's shield is 180 degrees and can at least catch incoming threats from slightly to the left or right.
The Onslaught is built to take on unfair fights and win. The Victory falters in an unfair fight. The Victory is severely vulnerable to EMP or HE flankers, bomber strikes from the side, and 2v1s in general. And the Victory's high DP cost (55!) means that you will be outnumbered. As a player, you can reliably avoid putting yourself in situations where the Victory's weaknesses are crippling, build the rest of your fleet around the Victory's weaknesses and strengths, and engage in a series of 1v1s with lesser ships that the Victory can chew up and spit out. This is a very powerful strategy, but ultimately so are many other endgame fleet-level strategies.