My issues with it were:
One of the main protagonists is overweight with chubby hamster cheeks... In the trenches, in WW1? That was my hopes of a realistic film dashed within the first 30 seconds.
The whole "saving the brother" aspect of the plot seems completely irrelevant to me, what about the other 1600 men, are they chopped liver? Feels like unnecessary melodrama.
The idea they would send a team of 2 to save 1600 men is ridiculous, they would send many different parties to increase the chance one would make it through, more Hollywood nonsense.
The second protagonist (who actually puts in a pretty impressive performance) is introduced to us as battle hardened, world weary and street wise compared to the apparent fresh faced other lad but this narrative is smashed when he faces a German sniper in combat and handles his Enfield like an old granny, only clearing the chamber a good 5 seconds after firing - Cringe.
The downed German pilot who after being saved from the burning wreck decided to knife one of our plucky heroes stuck out to me as odd, I suppose it's entirely possible but does fly in the face of everything I've ever been taught... Weren't the German WW1 pilots mostly wealthy aristocrats? Gentleman who fought with honour? The idea of one knifing the man who just saved his life seems a trite continuation of old "barbaric Hun" anti-German propaganda.
Next up is the 4 lorry convoy which happened to just be driving through no-man's land, how did they get there (the Ypres bypass?!
What are they doing there? We've just followed our boys through the deadliest journey of their lives traversing hellish swamps of dead bodies and booby trapped German trenches and there's a convoy who apparently got there without any issue whatsoever? Why didn't they just hitch a lift in the first place.
When our remaining protagonist takes a prolonged dip in the river his very special letter is amazingly unscathed, not a single run in the ink.
There's probably more nonsense I missed, the ending is a bit of a blur as the cinema was very hot and I'd switched off by then.