Recently I have been playing Mass Effect 1 again. It is distinctly different from the later entries in the series in a bunch of ways but I think the most important way is that it tries to present a real world you can interact with in all kinds of ways. ME2 and ME3 instead present rigid combat challenges you must overcome in a specific sequence with very restricted tactics. In theory I like the idea of an open world to explore but in practice there are a lot of issues with the added 'realism'.
First off there is the issue of retreat and zoning. In an open world there are buildings I can enter where enemies attack me and one totally reasonable response is to focus fire down a single enemy and then zone out of the building. The enemies can't leave the building so they reset but the dead one stays dead so I can zone back in again and repeat. This, obviously, does not adequately represent the response of real people to having armed invaders break into their residence and start shooting. I also regularly abuse my ability to hop in and out of my tank to fight enemies - while the tank is hidden behind a hill regenerating its shields I run around shooting at people with my pistol. This is actually an effective tactic but not one that increases immersion.
ME2 / ME3 and games like them have different immersion issues. If you can't win a fight by just being good at fighting there are no alternatives - every zone is just a twisty hallway with scripted fights and there is no way to get around anything. If you aren't winning your new strategy can generally be summed up as "Only shoot the enemies in the head." It does feel very strange indeed that I can't ever flank enemy positions, or wander over that hill to see what is there, or just bloody go back to my spaceship and blast them to cinders from orbit. In ME1 you can just fly away from a fight and come back later with new and bigger guns to blow them up if you really want to. Why they sit there unmoving waiting for you to return is a question not adequately answered.
So I love the idea of an open world where you can do anything but in practice it means that the players abuse mechanics like zoning, enemy pathing, and running away to beat challenges instead of just using 'legitimate' combat tactics. Without seriously controlling player behaviour there just isn't a decent way to build enemies that are immune to such tactics. WOW does it by having enemies instantly heal to full and sprint back home, immersion be damned. Most shooters just refuse to give you options to abuse and games like Skyrim just let people cheese the monsters out if they feel like it. I suspect that this is a problem without a solution. You just have to accept limited options, unrealistic behaviour, or overpowered cheese tactics and run with it. Avoiding all of them won't happen.