Meh, the assault cannon "only" takes 65% TUs, so there is some mobility in between shots. Even more on the production line "give it to 2-3 gals a turn" thing. And production of "very cheap ammunition". 5-6 radius kabooms is amazing, with a bit of movement left to do so. Bigger than that, with more TU's to do so? Meh, you can actually run this as a main gun. It doesn't just need a mortar position, it's just that you can run a forward one of them if you want to.
Actual Mortar? Nup, can't move, 94%TU fire, god-alone-knows how long to reload, and the explosion is too-big. You'll murderize way more than you want to. Grenade Launcher? Great and quick, very useful, and many versions of, but doesn't get awesome XPiratez-tech.
You'll just have to deal with bravery being a shooting stat for now (instead of getting vaguely molested by Hydra Cannons, as an actual "weapon upgrade"), but you got 40-69% out of that weapon with low-end tech anyway. You literally have "do a thingy, really well, with all-the-shells", but you can't do it really accurately. But you do essentially have plasma shells that blat armour, so there's that... You got them really early too. And training bravery isn't exactly a bad thing to do anyway).
There's also the major fact that heavy damage arcing weapons, even whilst kneeling, don't work all that well inside. It's a very useful thing on gameplay balance. But it is worthwhile blowing out the roof, from time to time.
(I'd find constant forced side-grades and busy-work from my basic hand cannon, musket, boarding gun, or cutlass or handle, to be just as annoying. I mean, I don't mind a bit of work, early in the tech tree, but after that it's just gravy, or up to me. It shouldn't be "necessary". And it's not. I can train my gals, quite well. So assault cannons aren't necessary either. But a lot of people forget they have "super-heavy bows" pretty early-on, with "siege-weapon level ammo" for them, able to be manufactured right now. Then say "however can I deal with this thingy, that is bothering me? It has all the stats!". Well, check what you've got in storage, I guess)
((But yeah, replace "Assault Cannon" with "Heavy Seige Bow" in your Mind, and "cannonball" with "arrow" likewise, and I think most people will realise how awesome it is to have a mobile heavy seige bow somewhere in their line-up))