Strictly following IIFYM neglects crucial elements such as the types of fats you are taking in, the types of carbohydrates you are ingesting, micronutrients, vitamins and minerals. Sure IIFYM can help maintain an aesthetic physique, but in the long run, it can be harmful to the body. Eating the wrong types of foods can cause increased inflammation in your body and interrupt the way the body functions, causing increased joint pain, degeneration and a host of other avoidable health issues.
Here are two great opposing examples of macro nutrition in bodybuilding and fitness from my personal life to explain my rationale. Body Builder 1 is an IFBB pro who follows a form of IIFYM in the offseason. He looks phenomenal every single day, but has never reached his full potential because his diet lacks certain essentials. He eats cupcakes, cookies, and anything he can get his hands on as long as he hits his macros for the day with the rest of his diet. This same bodybuilder has had kidney stones multiple times and who knows what other health issues are brewing and how long he has left in his competition days. On the other hand, Body Builder 2 is also an IFBB pro, not only eats clean 90-95% of the time through every year, he also emphasizes the quality of the food choices he makes. This guy is going to be competing in the Olympia Competition for many years to come, baring any major hindrance to his health.