Evan C’s review published on Letterboxd :
Sure, there's some competent cinematography, but the script and the editing have all the subtlety of a sledgehammer. For the first hour, the film painfully dances around an obvious "twist" that you hope and pray it will avoid, mostly because there's no way to introduce it smoothly, and because it's cliched and uninteresting. When the twist does come, the revelation is as awkward as you would expect, but to be fair, it creates a genre shift which briefly gives the film some worthwhile observations about the evil of nationalism and how war destroys families in all countries. However, those observations are beaten to a pulp just as badly as everything else when the location shifts from Germany to France. Because in the aftermath of WWI, both German and French people hated each other; it wasn't just one country directing hostile animosity toward the other, it was both. Therefore, we need about twenty contrived scenes of Germans being rude and passively aggressive toward French people, and vice versa.
The plot concerns a French soldier who shows up in a small German town and convinces a grieving family that he was friends with their son (the titular Frantz), and they were bitterly torn apart by the war. Frantz' fiance takes a special interest in this friend of her late beloved, because he knows so much about Frantz it's almost like having him back home. The soldier is naturally hiding a secret, and his real reason for being there is a total mystery, unless you think about about his obsession with Frantz and constant nervous antics for half a second. However, his presence causes Frantz' very nationalistic German father to have a change of heart, and Frantz' father gives a BIG SPEECH about how both countries lose sons in a war, and how we're not really that different from our enemies. The sentiment is nice (and true), but he only gets to that conclusion through careful manipulation and lying, which really rubs me the wrong way. It's one thing to withhold painful truths that people aren't ready to process, but to suggest that he would revert to being the worst of the nationalists if he learned the truth rather undermines the film's message.
Finally, I did appreciate the focus on art and music which brings out the shared humanity of everyone, but yet again, Ozon has to beat these ideas to death as well. Any moment which emphasizes putting aside our differences and focusing on the common things which make us human is highlighted by very notable shifts into color cinematography, because it is in these scenes that the characters are living life, which means they are full of joy and color, which we might not have know without the camera to remind us.