As some others have said, there was a lot of pertinent information left out of the series. The filmmakers admitted that the main intent wasn't to exonerate Avery or Dassey, but to shine a light on how messy the justice system can get.
There was so much fishy stuff going on from so many people. Based on some comments I've read from defense attorneys and prosecutors who weren't involved with the case but researched it, there was enough evidence to build a solid case for a conviction. If the Manitowoc police would have stayed out of it, like they were supposed to, they could have avoided a huge amount of trouble.
Then there was the conduct of some of the prosecution that muddied the waters even more, and the whole thing with Dassey was mindboggling. It's amazing his entire videotaped confession wasn't thrown out. A minor being interrogated without parents present by a PI hired by his own lawyer trying to get an admittance of guilt to use in court? How on earth is that justified as acting in the client's interest?