No-one wants 100% realism in a SFF movie. Otherwise we'd just look out the window. Historical movies, probably heading that direction but still not absolute.
What gets me is the deal breaker scene. The one where you're okay with aliens, powered armour, teleport, and so forth then they beslubber it up.
Probably a little overly specific but take this. Doctor Who recently had an episode with a newly introduced race that were dead funky warriors that wandered around killing everyone that bothered them. Called the Mire. Supposed to be terribly scary soldiers. Okay so far in a show with time travel and teleport all over the place. Then they have a scene where these dead scary warriors, on a mission to wipe out a Viking village, all cluster together in a small group on the road. Then they wander into a barn, still all clustered together. Then they get scared and run away. There's the deal breaker. This is something teenagers playing CoD wouldn't do. This is something obviously wrong. This is a Didn't Bother to do the Research moment.
The BBC is (in)famous for it though as they don't appear to ever have a military advisor (or someone who plays CoD or reads books even) on staff.
As you mentioned, immersiveness, the sort of thing above just drags me out of it. It's one thing where the moment is after the fact, you're walking home or at the pub and the thought his you - "hang on a minute". It's another when the thought hits you right there in the middle of the experience. You hit pause and wonder "what the beslubber was that?"
Wyddr - Have you ever read any of the Bolo series? 20,000+ tonne tanks, tracked and anti-grav, that operate as planetary assault/defence systems? The story is more about the morality and function of AI than war but a whole lot of fun. *Utterly* unfeasible but you're there in the moment.