Ibuprofen is generally considered safer than Acetaminophen BUT non-steroidal anti-inflammatory drugs (NSAIDs) like Ibuprofen can trigger dangerous stomach acid

Ibuprofen is generally considered safer than Acetaminophen BUT non-steroidal anti-inflammatory drugs (NSAIDs) like Ibuprofen can trigger dangerous stomach acid over production in some patients (source: finding myself in excruciating pain followed by a doctor pushing a camera probe down my throat to examine the damage when this happened to me). That's a minority of patients, obviously, but there are classes of patients who need to take Acetaminophen instead of NSAIDs for various reasons. Acetaminophen is similar to NSAIDs in many of its beneficial effects but is sufficiently different chemically as to not trigger many of the (rarer) negatives that NSAIDs can trigger.