I can only think that the reviewer who thought that this should have been more like The Hangover has little to no knowledge of the First World War, let alone of conditions in the trenches. Maybe diffusing unbearable situations with humour (and much of the humour that emanated from those trenches was actually blacker - and bluer - than that portrayed here) is one of those Very British Things. That was certainly brought up in this film, when the contrast was drawn between the Germans' "Hate Song" and the British soldier's humour and irreverence.
Personally, I really enjoyed the way that the sketches were shown: much about them was true to the time period and yet they were presented with a touch of the surreal to remind the viewer that what you saw on the screen was only a stylised representation of the written word.
I thought the actors were very good; the subject matter bittersweet, and overall it was an excellent representation of a fascinating story. There were one or two moments when I felt that the smooth, sarcastic, off the cuff exchanges between the major characters was a touch too smooth, but overall it fitted the period, the story and the way the screenwriters and directors chose to present it.
If you don't already know anything about The Wipers Times or about the Trench Warfare of the First World War then you may, indeed, not enjoy this as much as someone who does have a little bit of prior knowledge.