I find it surprising that people consider the onslaught weak - there have been several recent videos of them performing extremely well, including in the multi-ordo endurance killfests where they don't mount missiles at all. Here's one by draba for example vs 6 ordos:
https://fractalsoftworks.com/forum/index.php?topic=29311.0In terms of a firepower comparison between Conquest and Onslaught: Tldr: the difference is not very large in terms of gun offense.
On the one hand, the Conquest has much more dissipation. On the other hand, it doesn't have the
mounts to use that dissipation with high efficiency, so the difference isn't as large as it first appears. Gauss/Mjolnir/HVD/Mauler/Ion beams are the most common offensive weapons and they do a great job combined with Conquest mobility, but they aren't particularly efficient.
The onslaught otoh doesn't have the dissipation, but it does have the mounts, and I don't just mean the larges. It has 5 converging mediums (depending on arc, that's 5 forward but there are different numbers on the sides) and smalls that can potentially be range boosted (though I'm not 100% sold on the value of that). Even with some of those weapons as points defense, it can afford to use higher efficiency weapons than the Conquest can. Consider for example an HAC/Heph main armament - the onslaught will be firing with .9 f/d efficiency (or less considering tpcs are .6, but they miss a lot so I'm not sure what their accuracy adjusted efficiency is). This is kind of a 'middle' efficiency onslaught build, as it could be using higher.
A conquest going mjolinirs + HVD (brought up earlier) will be firing at 1.14 f/d. 1.26 f/d with an ion beam. The ratio of Conquest offensive efficiency to Onslaught offensive efficiency (1.14/.9=1.27) is really not far off from the ratio of flux between a Conquest (~1800 dissipation depending on build) and an Onslaught (~1400 dissipation depending on build): 1.29. Similarly with capacity: 17k vs 20k is in the Conquest's favor for offensive guns by raw number, but not if it is using lower efficiency weapons.
These are estimated numbers that depend heavily on build, so there is some ymmv, but I think the general point of 'Conquest can't use its dissipation as efficiently' holds. As a slight pre-empt of 'why don't conquests use hac/heph too?': they can but because they don't have a many medium mounts their ratio of kinetic:HE would be out of whack. Plus in that case they have to spend less on vents but still don't have more firepower, because they are capped at 2 large 2 mediums (+ a medium energy). It doesn't leverage the high dissipation that it pays so much for in terms of shield efficiency.
I'm not saying Conquests are bad btw - they are good ships and far, far more mobile than Onslaughts. Just also squishier and with very different optimal loadouts in terms of types of guns that means that their apparent firepower advantage from dissipation isn't accurate.