WARNING: "Hacksaw Ridge/American Sniper" spoilers below
For the most part, the film actually does do a decent job of showing how nasty not only war but violence in general ends up being (even in the pre-war section of the film you still get moments to this effect - Desmond almost killing his brother with a brick, his psychologically unstable WWI veteran dad whose attempts at disciplining his children come across as physical abuse, the bit where Desmond saves the car accident victim with the damaged leg). The vast majority of the sequence involving Hacksaw Ridge itself manages to make the combat more disturbing than "exciting" and the chaos is so overwhelming that it gets hard to tell who is "winning" or not because of how many people are getting brutally maimed or killed. In the middle of all this, you have Desmond himself, who just wants nothing more than to save his comrades (and even the odd Japanese soldier) without fighting anyone, and it is his determination in the face of such horrifying circumstances (where he is even forced to break his vigourously-defended "never hold a gun" rule because it ends up being the only way he can carry someone to safety) that comes across as the real heroism. In this context, his status as an American soldier is circumstantial compared to his status as a human being with the capacity for selflessness and compassion even in the face of armed conflict.
The problem is that, after Desmond manages to save as many of the troops as he can (which would seem to be the true climax of his arc and, by extension, the film's narrative), the film goes on to show the U.S. forces regrouping and taking the ridge from the occupying Japanese forces. It stands out because of how heavily it contradicts the work that the rest of the film has put into making violence look horrible all for the sake of a "triumphant" finale (complete with a meme-worthy "grenade kick" moment for Desmond himself). Before, the Japanese soldiers being barely-glimpsed forces of destruction merely came across as an extension of the war-is-hell approach the film was going for, but with this scene they become faceless cannon fodder while the leaders' ritual suicides are even captured in stylised slow-motion. I get the impression that showing the final battle is supposed to indicate how Desmond's unconventional approach to soldiering (which he was mocked and interrogated over for much of the second act) still ended up making a greater difference than anyone could have anticipated, but it instead seems better at overwhelming his contribution by emphasising the U.S. victory over his ability to stick to his morals no matter what the war threw at him. If the film is Desmond's story, then the U.S. winning the battle seems like it should be besides the point. I know that's what actually happened, but to depict it the way they did really flew in the face what the rest of the film was clearly doing.
For comparison's sake, consider the ending to American Sniper (a film which I disliked much more than Hacksaw Ridge). The film has seen Chris Kyle sign up for the SEALs and go on multiple tours of the Middle East, though he is shown to be experiencing PTSD during his downtime as a result. To this end, he also cares about helping out other veterans with PTSD, and the final scene of the film is him leaving his house to spend time with one such veteran. A bluntly-worded title card then tells us that said veteran ended up shooting and killing Kyle. This may just be based on a lack of information about what actually happened between the two, but a move like that does a good job of getting across the shockingly sudden nature of what happened because showing it would have come across as the kind of dramatic irony that the rest of the film pointedly avoided.
For the most part, the film actually does do a decent job of showing how nasty not only war but violence in general ends up being (even in the pre-war section of the film you still get moments to this effect - Desmond almost killing his brother with a brick, his psychologically unstable WWI veteran dad whose attempts at disciplining his children come across as physical abuse, the bit where Desmond saves the car accident victim with the damaged leg). The vast majority of the sequence involving Hacksaw Ridge itself manages to make the combat more disturbing than "exciting" and the chaos is so overwhelming that it gets hard to tell who is "winning" or not because of how many people are getting brutally maimed or killed. In the middle of all this, you have Desmond himself, who just wants nothing more than to save his comrades (and even the odd Japanese soldier) without fighting anyone, and it is his determination in the face of such horrifying circumstances (where he is even forced to break his vigourously-defended "never hold a gun" rule because it ends up being the only way he can carry someone to safety) that comes across as the real heroism. In this context, his status as an American soldier is circumstantial compared to his status as a human being with the capacity for selflessness and compassion even in the face of armed conflict.
The problem is that, after Desmond manages to save as many of the troops as he can (which would seem to be the true climax of his arc and, by extension, the film's narrative), the film goes on to show the U.S. forces regrouping and taking the ridge from the occupying Japanese forces. It stands out because of how heavily it contradicts the work that the rest of the film has put into making violence look horrible all for the sake of a "triumphant" finale (complete with a meme-worthy "grenade kick" moment for Desmond himself). Before, the Japanese soldiers being barely-glimpsed forces of destruction merely came across as an extension of the war-is-hell approach the film was going for, but with this scene they become faceless cannon fodder while the leaders' ritual suicides are even captured in stylised slow-motion. I get the impression that showing the final battle is supposed to indicate how Desmond's unconventional approach to soldiering (which he was mocked and interrogated over for much of the second act) still ended up making a greater difference than anyone could have anticipated, but it instead seems better at overwhelming his contribution by emphasising the U.S. victory over his ability to stick to his morals no matter what the war threw at him. If the film is Desmond's story, then the U.S. winning the battle seems like it should be besides the point. I know that's what actually happened, but to depict it the way they did really flew in the face what the rest of the film was clearly doing.
For comparison's sake, consider the ending to American Sniper (a film which I disliked much more than Hacksaw Ridge). The film has seen Chris Kyle sign up for the SEALs and go on multiple tours of the Middle East, though he is shown to be experiencing PTSD during his downtime as a result. To this end, he also cares about helping out other veterans with PTSD, and the final scene of the film is him leaving his house to spend time with one such veteran. A bluntly-worded title card then tells us that said veteran ended up shooting and killing Kyle. This may just be based on a lack of information about what actually happened between the two, but a move like that does a good job of getting across the shockingly sudden nature of what happened because showing it would have come across as the kind of dramatic irony that the rest of the film pointedly avoided.