Well, in XCOM:EW/EU all the tech was pretty accessible to imagination. You handled most of the game with ballistic weapons and explosives, followed by direct energy weapons. The aliens had weird plasma weapons and you'd later get them too, but that was pretty much the strangest thing. That, and all the auxiliary equipment, always followed simple patterns that are known in the real world. Obviously within an abstract framework that most games use (I somewhat hate the concept of hit points), but we're so used to that...anyways, in The Long War you can modify your weapons in a number of ways: Accuracy, magazine size, AP ammo and similar things. All reasonable enough. There's also some stranger stuff like boosting damage on beam weapons, or increasing critical hit chance...but it's still something proportional, and fitting to the mental framework of the whole system.
But firearms that are guaranteed to deal damage, or have a situation-independent probability of instantly killing anything from the smallest to the largest foe?
That kind of mathematical black magic really itches. Doubly so if the items in question are a "repeater" and a "stock". The what? How is that supposed to work? Is this the game design way of saying "phug anything resembling coherent world design, have some game mechanics!"? Even discounting the immersion factor, how is this supposed to work out for game mechanics? These bonuses are completely disproportionate to the nature of the weapon, and put immense value on just being able to fire as often as possible.
And I still can't get over the implications of such technology. Why train expert soldiers, why give them a variety of weapons? Why not make weapons with larger ammo pools and higher rates of fire in order to better exploit the insta-kill potential? The projectiles don't have to individually hit or penetrate enemy armour, it'll be sufficient for them to go in the approximate direction of the enemy. It's pointless to have big guns or accurate shooters, so just send out a hundred redshirts in a 19th century line formation and have them spray wildly in any direction the enemy may be in.
Think further! Why have artillery, tanks, cruise missiles and guided bombs? The wars of the future will be fought with clouds of tiny bullet-lets...and stocks. And "repeaters".
Alright, I know it's just a game. But that's silly.
I just had a skim through the XCOM2 wikis, and I can already see that I won't be liking it. Maybe it's for the wrong reasons, but the realistic-ish immersion factor provided by The Long War isn't there, whereas a much more game-y "just because" mindset seems to be more prevalent. Shame.
Well, you do you, I guess. I understand the weird disconnect between the names of mods and their function, like how a stock adds damage on a miss, but that doesn't affect immersion for me at all. In terms of the actual mechanics, I don't see how a chance to insta-kill is any less immersive than a critical hit. You got a good shot off and damaged something that destroyed the enemy in a single shot.
I mean, things in XCOM have always been way more durable than they should be, what with the hit points you mentioned hating. You shoot a guy point blank, he's not going to just stand there and take it, he's going to die. In one shot. As far as I can tell without knowing the devs' intent, that's what the insta-kill represents. Get a good shot off and hit a real tank in its ammunition load and it could easily be rendered combat-innefective, lose its crew, or otherwise be destroyed in one hit. The weird stuff like insta-kill being triggered by poison's DoT is indeed strange, but that is a very specific and fringe case.
The modded stocks could be explained that they make the weapon easier to handle, improving the soldier's ability to hit in ways that inflict superficial wounds even when they fail to hit in a way that does "significant" damage. In this sense it's the inverse of the dodge mechanic.
As for why we don't just give everyone these things, it's because we can't. XCOM doesn't
have a hundred redshirts to throw at the enemy. Sending that many rookies at any mission would probably be a lot more effective than the kill-teams we have, but that's impossible. Like I said, this is why I think XCOM 2 is more immersive than EU/EW. In the new game our limitations make sense, but in the first one, when we're allegedly an organization backed by the UN or whatever, we should be able to do as you say and drown them in bodies. And that game didn't even have the mods you say wreck the immersion, which we also don't mass-produce because they're alien technology that we don't have the resources or the knowledge to make, so we have to steal their good gear from them.
The only thing I agree with you about is what the mods literally are. A repeater conferring this chance doesn't make much sense, it would be more logical to be conferred by a soldier ability, a type of ammo, or something else. And maybe stuff like the final bosses being susceptible to these shouldn't be the case from a gameplay perspective, as it's fairly anticlimactic, but I would view that as even more immersion breaking than the current system. It's not uncommon for bosses to be immune to many effects in games, because that would often times allow silly amounts of cheese, but arbitrary immunity to effects doesn't make a lot of sense.