There is a difference to learning and making statements, some dewormers (not allowed to mention brands) damage a lot of Protozoa and bacteria post caecum, which can allow the tract to become overrun. Acute side effects are seen as simple excessive motility of the gut, but occasionally there have been reports of more severe side effects. I weighed up the cost of risk and because my dog doesn't have behaviour which may encourage parasites I decided it wasn't worth doing him. I'm demonstrating why not all dogs need to be routinely dewormed, I really do hope you agree on that!