The wooden delivery of dialogue (certainly in the first hour or so) kept breaking immersion and the fact they couldn't re-shoot as easily for minor tweaks really compromised it IMO.
Another thing that broke immersion is that I couldn't suspend enough belief for the fact he covered maybe just over a mile over the whole film? Obviously that was true in real life, but the long shot meant nothing was broken up enough to give the sense of distance, certainly not the feeling that the 12 hours or so would have been an ambitious undertaking. Without the K.O from the sniper he would have made it in well under 2 hours.
A good example of that is the truck convoy bit, they literally took him about 100 meters... he could have walked that and saved himself a shed load of time.
The underlying awareness it was such a short excursion also started to make random events feel entirely unlikely and a little bit Playstation FPS, they were like 200 meters apart.
By the time he's running across the trenches at the end and the explosions looked like MSPaint, I just got annoyed.