Protein shakes are simply a concentrated, convenient protein source. They can help athletes, busy people, older adults, vegetarians, and those recovering from illness or surgery hit their targets when whole foods are impractical. There is nothing inherently unhealthy about them when used to fill a genuine gap.
Downsides are mostly practical: they can add unwanted calories and sugar, some products contain large amounts of additives or sweeteners, and quality varies because supplements are loosely regulated in many regions. Whole foods also bring fibre, vitamins and minerals that powders lack.
People with kidney disease, certain metabolic conditions, or milk-protein allergy or lactose intolerance should choose carefully or avoid some products. Check the label for added sugars and reputable third-party testing, and ask a doctor or dietitian if you have a health condition before relying on shakes.