Judges and jurors are just as human as the rest of us, and they make mistakes. And the trial process, long and complex, provides plenty of opportunity for mistakes to be made. This breeding ground of error, by itself, does not cause appellate courts much concern. Thus, the biggest hurdle is not showing that a mistake was made, but that your client lost as a direct result of a particular error.
Mistakes made by a judge are generally easier to attack on appeal than mistakes made by a jury.
Juries decide questions of fact, and their verdict frequently involves judging the relative credibility of witnesses. Assuming there is some evidence in the record to support a jury's verdict, appellate courts won't second-guess a jury, even if the appellate judges would draw different conclusions from the same set of facts. Errors of law made by a judge are more likely to result in a successful outcome on appeal.