How serious is a diaphragmatic hernia?

Mostly not serious: The majority of diaphgramatic hernias are referred to as paraesophageal hernias, and the majority of these are small. Many individuals actually have small "hiatal" hernias and are asymptomatic from them. If the hernia, however, is large and symptomatic, then it can be serious causing obstruction or even necrosis of the organs herniated through the defect. This is less common though.

Can be serious: It depends on the extent of lung damage.