It reduces a complex problem into a simplistic slogan that's then used by many people to hate and judge fat people. "You're fat, just eat less stupid fatty".
It ignores gut microbiome. Here's a story about a bug that makes it harder to gain weight: http://www.bbc.co.uk/news/health-22458428 and another bit of research about people who are obese: http://www.nature.com/nature/journal/v444/n7122/abs/nature05...
It ignores "satiety". It ignores all the psycho-social stuff going on around food.
It ignores changed metabolic pathways (one of the reasons people who take antipsychotic medications are overweight is because of the changes to their metabolism: https://www.ncbi.nlm.nih.gov/pmc/articles/PMC487012/)