I didn't even buy the backward engineering concept to begin with becuase if you took a truckload of current-day technology and dumped it in 1950, they didn't have the technology to be able backward engineer it, and that was in peacetime without most of the infrastructure blown to bits by aliens. The alien stuff the humans retro-engineered clearly relied on miniaturization and other processes that simply would have been unobtainable. That's what makes retro sci-fi so hard. Sci-fi writers typical imagine aliens with very, very advanced versions of what we can do today, which is why then imagining reverse-engineering isn't so far-fetched. What the Resistance writers did is have 1950s characters reverse-engineer the imaginations 2000s sci-fi writers, which just plain doesn't work. I think it could have worked if the Chimera used organic weapons (I could more easily imagine 1950s scientists capturing an alien bio-gun, breeding it, and mounting it on a jeep than I could imagine them building tiny radio transmitters to put in bullets), but instead they're using very, very advanced manufacturing and computers.
But my biggest beef isn't with that, it's with the 100% human weapons. M5A2 Carbine is functionally an M16 with an M203, a thing that didn't exist until the late 1960s. Thing is, we already know what the US would have had in 1951, because that's when the Korean War happened: M1 Garands, M1 carbines and rifle grenades. There's simply not enough time between the end of WW2 and the first Resistance game for military technology to have advanced that far. And we know what was in the design pipeline for the West: the FAL and the M14.
Furthermore, the dedicated sniper rifle wasn't even a concept in the 1950s. Until the 1960s, snipers simply used very high-quality battle rifles fitted with optics. In the Korean war, Western snipers used Springfields and Enfields, rifles that predated WW1. In Resistance, the L23 Fareye is basically a Dragunov with an M14 suppressor---it is a 1960s gun, not a 1950s gun.
Also, no, no minguns in 1951. Just...no.
The reason I go off about this is that I would actually like to play a first-person shooter set after WW2, but before 1978!!! But the game industry simply can't bring itself to do that. Gameplay conventions of the genre are firmly rooted in the 1980s and later, so what developers inevitably do is wrap modern technology in wood and steel, play Elvis in the background, and call that "the 1960s."
And don't get me started on Black Ops. What an utterly wasted opportunity that was.