Writing from Italy: this movie was indeed strongly criticized for its rethoric, which was read by some as courting the extreme right, or the "law and order" field, even though the movie was reportedly filmed to expose hate and its consequences.
I think the movie is a complex one. The policemen are indeed bastards, it's difficult to argue the movie really presents their point of view. Maybe the point of view of that kid who gets into police "because it's an honest job".
The two fathers are terrible parents, I don't believe what you see is really MRA from our point of view. A father who leaves his kid alone to go beat some immigrants up does deserve to get his child taken away.
Yes, the movie is filled with anti immigrant rethoric. However ... Are the neo Nazis there the good guys? Is the political right guy the good guy? Is the Tunisian immigrant who occupies an apartment in a housing project the bad guy?
Yes, this movie became popular with angry young people from the Italian big cities, children of the housing projects who have incarceration rates similar to black people in America and who use soccer hooliganism as a way to express their anger. Unlike America, cops in here come from the same exact background. Unlike America, people see the State as the bad guy, because they think the state should provide (jobs, housing, care). Like America, people feel easy to direct their anger towards immigrants.
But I think it's difficult to see this movie with American eyes. It's best seen from the eyes of a young Italian who lives in a big city. Even here, the question "Is ACAB a rightist movie?" didn't find an unequivocal answer yet.