They were fair. Just because evidence was so overwhelming and there was little they could say to justify their acts does not make it unfair.
What makes the trials unfair was how they were conducted. All judges came from Allies, not neutral countries. Effectively these people were judged by their prosecuters. If the trials were fair and objective, then the judges would have came from neutral countries like Switzerland and Sweden. The intent of the trials were to humiliate and execute Nazis, not to seek truth or justice.
Consider
Goering. Goering never supported an aggressive war of expansion and its a stretch to say he was an architect of Nazi crimes against humanity like the extermination of undesirables. In 1939, he argued against invading Poland and for working through international diplomacy (backed up by threats of military force) to take parts of Eastern Europe. His position was overruled by Hitler and other Nazis who sought total war to conquer all of Europe. I'm not convinced that Goering believed in the Master Race, but he did support the use of slave labor. The evidence is inclusive that he supported death camps. While Goering is hardly an Oskar Schindler, he did intervene to prevent some undesirables from being sent to the extermination camps. For example he protected his deputy who was Jewish. There are other examples where his anti-Nazi brother in law got him to intervene occasionally to save people's lives. Goering issued a written order in 1941 for the "complete solution of the Jewish Question" and passed responsibility to the SS. He never authorized the mass exterminations which started almost two years later. Even after Goering was sentenced to death and had nothing to loose either way, he still denied ordering or supporting mass exterminations. Given his answers to other questions where he admits to committing war crimes, I find it difficult to believe he lied to the end about the mass exterminations.
I've read his interviews and the guy was arrogant, obnoxious, brilliant and in the end, open and honest. His worst crimes were similar to Bomber Harris. He ordered the destruction of cities and targeted civilians. If he fought with the same record on the Allied side, he would have been decorated like Harris. But he was on the losing side so he was executed. Hardly fair or objective.
BTW, I am not sympathetic to Goering or Bomber Harris. I believe all war criminals should be held responsible for their crimes including Goering. But I doubt Goering had much to do with the mass exterminations. He was mostly concerned with the air war. He was no idiot, so he probably was aware that people were being exterminated and did little to oppose the policy, just like many other German officers and civilians who were not held accountable for their actions/inactions.
Karl Dönitz, head of the Navy conducted a campaign of submarine warfare which was copied by the Americans from the day they entered the war. A project at Yale University examined his case here:
The Avalon Project : Judgment : Doenitz
I don't see evidence that Donitz violated international law. As soon as merchant vessels became armed and carried arm shipments for war, they became legitimate targets.