Hiatal hernia: other than gerd (gastroesophageal reflux disease)/heartburn, what other effects might it cause? is it dangerous if left untreated?

Depends: Large hiatal hernias (or paraesophageal) hernias can cause chest pain, difficulty swallowing, and severe anemia from chronic bleeding. Only rarely do they cause life threatening problems such as gangrene. They are however frequently misunderstood as the cause of symptoms, so if a large hiatal or paraesophageal hernia is the diagnosis, see a general or thoracic surgeon with experience in lap repair.