Granted, the numerous chance encounters of the lead characters may be unlikely but they are an acceptable plot device. What can't be seriously disputed is that the mini-series takes great pains to put the audience in the shoes of the five young Berliners on their journey through the madness of total ideological war. The dominant question looming in the background is not so much the well known question, asked in the comfortable situation of a stable post-war order How could you be such a criminal tool of Hitler's genocidal war?" but rather Damn, what would I have done in their position and where do I take the smug conviction from that I would have done so much better?". The overall approach is not a conversation stopper between the generations but an outstretched hand, not a tone of indictment and condemnation but one of empathy.
I came across two major groups of audiences who got all worked-up and downright mad about the show: German internet-Nazis and patriotic Poles. The former were foaming at the mouth about just another installment of guilt-worship and defilement of the supposedly heroic and noble German soldier of WWII (and the millions of German civilians who got killed, raped and expelled from their homelands). The latter were upset that a German show about WWII in the east doesn't center on the suffering of Poles at the hands of Germans and even portrays Polish civilians and partisans as ardent antisemites.
Both camps, even the internet Nazis, have some points, I believe. When Friedhelm says in the first part that the Russians are learning from us" about atrocities, it's a fair objection to point out that Stalin's mass-murdering terror regime in fact didn't need any lessons from the German invaders about committing atrocities, be it politically motivated mass-murder, genocide by famine, ethnic cleansing, etc. At the time the film starts, Stalin and his countless henchmen still had much more blood on their hands than the Nazis and their helpers – which was going to change, however For a good account on this, read Timothy Snyder's Bloodlands).
By the way, a legal fact which is apparently too unpopular nowadays to be mentioned ever is that customary international law at the time of WWII allowed quite far-reaching reprisal actions of occupying forces when attacked by irregular forces. Even the US Army field manual of 1937 deals in detail with the accepted practice of hostage shooting and the burning of villages as reprisals Collective punishment had been commonplace in the soviet union since the revolution, and the British applied collective punishment in their colonial rearguard fights even throughout the 1950s. (Sadly, that list is far from complete: https://en.wikipedia.org/wiki/Collective_punishment). That doesn't undo German atrocities but it's not helpful either to view those completely out of historical context.
Polish criticims, as mirrored in several reviews here, centers on the fact that the show depicts the war against the Soviet Union and therefore leaves the Polish campaign of '39 and the subsequent German occupation of Poland largely out of the picture. What makes countless Poles then totally snap is the double dip of being once again portrayed as antisemites (remember the bitter reactions in Poland to a brief scene in Spielberg's Schindler's List or to the favorable reception in the US of books by the Polish-born author Jan Gross) and then, of all people, by Germans. While I believe that the subject of antisemitism among Poles could have been portrayed in a more balanced fashion, especially by blaming someone else than the AK who evidently helped Jews and even supplied weapons for the 1943 Warsaw Ghetto uprising, I think it's time people in Poland start to accept that todays's Germans have a right as anyone else on the planet to include Polish antisemitism in their renderings of history. As for brave Polish resistance to the Nazis not having gotten more screen time, it's the decision of the makers of the show what their story focuses on. I was also disappointed when AMC's Hell on Wheels reduced Plains Indians to mere plot devices but I guess that I just have to live with it that Hell on Wheels is about people building a transcontinental railway and not about the plight of the ethnically cleansed Indians. Same applies here.
Summing up, if you have seen enough comic-book Nazi villains on screen and are curious about the German experience in WWII, this mini-series is a pretty well-made, honest and quite engrossing attempt.