I found Hacksaw Ridge’s battle scenes bizarrely unrealistic.
abel29a has already pointed out the “gasoline bomb” phenomenon, where apparently the battleship guns were firing giant bags of gasoline. Artillery doesn’t look like that.
More pertinently, though, is that the impression one got of the battle is that it happened in a very small space. Guys were basically shoulder to shoulder; it felt as if the Americans and Japanese were fighting over a space about the size of a track and field stadium’s playing surface. Consequently, in a weird way, the battle feels almost indoors, and seems much more constricted than, say, the epic “Battle of Stirling Bridge” scene Gibson directed in "“Braveheart” (that battle scene bore no resemlbance whatsoever to the real thing.)
Soldiers fighting should to shoulder die very quickly, in fact. Modern warfare tends to be spread out a bit more than you saw in that movie, though hand to hand fighting did take place.
It is also worth noting that the battle included tanks; indeed, at one point Desmond Doss (the protagonist of the film) had to hide from a tank.
I could pick apart the movie a thousand other ways - for one, Doss and his unit are implied to be very green, when in fact they had seen lots of action elsewhere. But the impression of the battle is one of a very confined space where gross things happen; battlefields are bigger than that. Things also don’t happen that fast, of course; the battle for the Maeda Escarpment, aka Hacksaw Ridge, went on for days, and there’s a lot of pauses in battles as soldiers regroup and try to call down artillery.
What is definitely true is the casualty rate. Both sides were devastated; American forces suffered heavy losses, and most men did not escape without at least serious injury. Japanese losses were almost total. Ninety percent of all Japanese troops on the island, of whom there were almost 90,000, were killed. Only 7,000 were taken prisoner.