Versatile stories based on real events have a vast "deficiency" - most viewers know what is going to happen and how it all ends. Therefore, screenwriters and filmmakers should focus on other aspects, find some new angles or artistic supplements to make the creation an interesting and aggregated whole.
As for the miniseries in question, in my opinion, the task became often too complex. The three rather long episodes, dealing with different aspects of the "case", are too secluded, intertwining has flaws, and following is not equally interesting. The 1st episode is far more strongest and exciting, the 2nd is the weakest, being a story of Turks with tragic fate where NSU is mentioned only in the background. The 3rd part is hectic and full of conspiracy theories, and flashbacks make it all more confusing rather than assisting. As I was unaware of the events, I had additional thrill in the beginning, but soon I began to focus more on acting (several pleasant performances, no unnecessary "jutting out") and living-events proceeding from the fall of the Berlin Wall.
Due to the above-mentioned, I would give 7 points in general, with Episode 1 - yet 8 points. All in all, Mitten in Deutschland: NSU is not a weak creation, but not at the level of e.g. Deutschland'83 or Weissensee.