It all about the right tool/damage type for the situation rezaf, getting through the game with only one weapon for all situations is not the way its designed.
It's not about wanting to use "one (type of ) weapon". I have a pretty broad range of stuff, starting with Bows and some projectile smallarms (I never use them, really) to Laser Pistols, Rifles and then Assault Lasers, all sorts of grenades and explosives, Mortars and Bazookas, still some Shotguns (the Laslock Shotgun is especially versatile) and a couple of melee and stun weapons (I also hardly ever use)... I trew out most other stuff, because most of it just is pretty useless.
I just mentioned the Assault Laser because at least it CAN bring down those enemies in the end. I would expect specialized weapons to do their job - noone should survive being hit in the face by an anti-tank rifle. Sadly, X-Com has no physics, otherwise the victim should be killed by being thrown around alone, but ... yeah, this weapon is regularly shrugged off by Marsec Bodyguards.
The Heavy Slug Thrower sounds like it should have some oomph, but it reliably underperforms despite massive TU consumption.
Grenades? Mortar fire? Explosives? Specialized projectile weapons? None of that stuff works.
I CAN work my way around it 75% of the time, but it just server to drag out missions.
And this is just one area of the mod. I really love that there's so much stuff in XPiratez (even though not all of it suits my taste), but there should be much more "gating". I should not have the Stone Axe, Javelins, some Musket variant, a Laser Rifle and a Plasma Pistol all available as research projects at the same time. $0.02 and all, sure.
Btw., NuXCom is great for one playthrough and then forgettable. (Xenonauts is just X-Com but not quite as good, and who needs that?)