Yeah.. that works in Piratez where the gals are a bunch of wackos escaped from some weird experiment or picked up from the deepest primitive slums, but it makes little sense for any kind of police or para-military organization to need to interrogate people to learn about AK-47 or machetes or axes, or to need to look at a physical one to know what it does. Players keep complaining about it because it is jarring and not fun. Maybe making USH useful is nice/fun for you, but keeping it truly useless and non-obtrusive is much better for the player.
This is also very setting dependent. One can get excited in Piratez when the gals pick up an AK-47 and "figure it out" because they're making progress from the cavegals you start with by rediscovering all the weapons of history in a post-apocalyptic world (especially since you have written the description in such a way that they're a reward in themselves for how enjoyable they are to read, regardless of weapon utility). In XCom or XCom files? Nope, it just makes you feel like you're in charge of the most retarded, crippled organization 20th century Earth could possibly put together to investigate/fight aliens. XCom is about 20th century people discovering new alien stuff, not rediscovering 20th century weapons that are currently in use and not at all forgotten (and where description have to be much more sanitized given the institutional/military setting unlike in Piratez, so there's much less potential for cool descriptions being the actual reward). As such, your use #2 doesn't work in XCF, or maybe it does for you, but it is unpleasant to many players, enough that it is probably the most recurring complain about the mod (regardless of if the mention is of either AK-47, axes, hammers, etc. They're all USH).
Use #1 and #3 are very true and it is nice for the setting that rebels and cults are armed with AK-47, machetes and axes. I certainly wouldn't exchange that for everyone being armed with a "Rifle". It's also nice that, should you fancy having a Japanese katana warrior, a Russian AK-47 dude or something, you can. But that's it for the value of USH. As such, the mechanic I am proposing gets rid of #2 because it doesn't fit while to enable #1 and #3 in as non-obtrusive a manner as possible.
or hidden behind bulk researches that will forever clutter the research que, so the player will finally research them, but not to collect a collectible, but to finally get rid of that shit, which is a very different feeling, again.
Yeah, discovering boring stuff is boring. I totally agree. If you think discovering 20 at a time still isn't exciting, how is discovering them one by one more exciting? I can tell you as a player of XCF, and others have said it as well: It is even more boring to discover the 20th useless item than it is the first one. It's even more "Oh! Come on!" and eye rolling worthy. So let's do away with the "achievement"-style accomplishment of "discovering" what an AK-47 is in the 20th century, discover all the USH at once and then move on to discovering actually interesting stuff.
You want to clutter interrogations so people don't get all the important bits too quickly? Have some text about whatever cult so the cultists can ramble on about that after their interrogation. The "Notable Cultists" results are an awesome example. You didn't get
anything weapon-wise, but you learn a little bit about stuff, and the "Interpol notified" line makes you feel useful even though it's just a couple points because it feels like you are helping fighting crime/the cults. I'll take that over machetes/katanas/etc. any time! Then it feels like you're learning and progressing in your investigation of the cult, while enjoying something
creative in the awesome setting that Solar created. I'd
love to know even more about all the crazy cults and their backstory and every bit of into is enjoyable because it's like reading a page of a novel, or from 40k codex before they got all crazy. Learning about an intelligently designed, creative and new setting is cool. Learning about AK-47 and axes when playing a 20th century organization just plain isn't.
Furthermore, in XCF, most of the USH research comes from the actual items being researched, not from interrogations, so neither the "cluttering the research list" argument nor the "delaying interrogations" argument really work. I'm suggesting a few projects to unlock all of the USH at once. The current state is that every weapon you bring back from a mission shows up in the research list as a thing to be researched and stays there
cluttering the research project list until you finally give in and dedicated a scientist to research machetes and another one for axes so you don't have to see them in the list anymore (and just in case not researching the machete will prevent you from inventing the "Machete of plant slaughtering" that is needed to win against the Treemen, or something like that, because who knows what Solarius came up with and you better cover everything. Or you peak at the techtree and realize it doesn't lead anywhere, but still do it because after 3 years of in game time, it's about time you got rid of that stupid project). Good that you bring this cluttering up, because that's another argument in favour of my fix (1-2, maybe 3-4 projects if you were to do the same for general use pistols and rifles, instead of one per item cluttering the research list).
