If there has been structural error, reversal will occur even if there was no trial objection if the appellate court agrees that structural error occurred. Also, even if there has been an objection at trial, and the trial judge erred, the appellate court must decide if the error was harmful. If the error was harmless, that is, did not really prejudice the appellant in any way, no appellate court will reverse the judgment based on that error. Again, a very few kinds of errors do not lend themselves to harmless error analysis. That is, a cunning lawyer will throw a mass of issues at an appellate court, wave a magic wand, and the judgment is reversed.
Real life is different.
Appellate courts try their hardest to uphold judgments, because there is a great social interest in the finality of judgments. Instead of looking for ways to reverse, appellate judges look for ways to affirm. The appeals that win are those which show the court that something fundamentally unfair or improper happened in the trial court, and the appellate court most likely will then reverse the judgment in order to preserve the integrity of our system of law. It is counsel's job to identify the errors which took place in the trial court, argue that they were in fact errors, and that the appellant was harmed as a result. A skilled appellate lawyer will do everything possible to persuade the appellate court that justice will be best served by reversing or modifying the trial court judgment.