One thing I always thought weird about the original XCom was that human researchers could reproduce alien technology so easily. Yeah, you hardly ever need to build Plasma rifles or their clips if you scavenged them from aliens, but you could build them, along with pretty much everything else.
When you think things through, alien tech scavenging and reuse is a major flaw in gameplay in UFO and it's sequels.
Consider: You research Heavy Plasma and that's it; you basically never ever need to manufacture personal weapons anymore; except maybe for small production runs of Stun Bombs for the Small Launcher since you now have weapons piling up after each mission.
It makes more sense if you cannot use alien weapons because they are:
A.) Designed for alien hands and musculature.
B.) The weapon itself has a wireless RFID chip that interfaces with a RFID chip in each alien soldier.
Thus explaining why your soldiers can't just pick up a heavy plasma and push the firing stud and fire it; thus requiring research into how to overcome the RFID encryption to 'unlock' the weapon.
Thus, you would then need to "remanufacture" alien weapons into human-useable weapons, e.g. replace the alien handgrips with human ones, remove the alien RFID lockout, and add other useable indicators. Maybe the aliens use psionics to tell the soldier that there are only five rounds left in the magazine; requiring the design of a LCD panel on the side of the weapon to make it useable to humans, etc.
And all this remanufacturing has a non-trivial failure rate -- e.g. you need to have three heavy plasmas to make a remanufactured heavy plasma; and the gun has a random chance of breaking after each mission.
I'm not quite sure how well that last point would be received. There's a lot of stuff that makes sense -- on paper -- in Game Design. It would add a lot of overhead to the player if they had to go: "welp, I had five guns broken from my last shootout, guess I need to remanufacture more guns."