There's much good about this movie, starting with Ron Stallworth's incredible deception of the Klu Klux Klan. Racism in all its ugliness is powerfully shown. There's a lot of humor at the expense of some really dumb people.
Unfortunately, there's a lot wrong with the movie too. Most of this is because the director embellished the true story. I'm not a big fan of directors tinkering with what really happened in order to add their own touch, and then still claim "based on a true story".
The result of the tinkering is a very uneven movie, particularly in the apparently "easy" parts of infiltrating the KKK and the "hard" parts where things go wrong. The "easy" parts are, remarkably, mostly the true story. Apparently this wasn't dramatic enough, so a lot of fictional "hard" parts were added to build tension including whole characters and situations. That's bad enough, but the added parts often made no sense, such as having no real origin (like one character's intense suspicions) and no resolution to the dilemma presented - they just seem to go away, are forgotten or have no effect on the inevitable story arc. Many seem to have been thrown in only to make already duped people look even more ridiculous.
The characters themselves are, with a few exceptions, just caricatures. It's not hard to figure out what's next since they do exactly what you expect.
Eventually the movie just got boring since it all moved to an inevitable and very easy to see end. Ultimately, the movie is maybe an hour of an amazing true and humorous story marred by over an hour of superfluous and poorly executed fiction.