Her weakness is very explicitly her naivete.
This is not a weakness in the film. It could have been, but it wasn’t. When she goes to the front, she is told she cannot cross No Man’s Land. She doesn’t understand the kind of modern artillery firepower she is up against. She does it anyway and succeeds. She’s told there is a gas that can end civilization and kill millions of people. She has probably never encountered something like that before. She walks right into a cloud of it and lives anyway.
At no point did her naivety cost her anything. It was a nice storyline that I was digging, but it was never a weakness.