Is weight gain always about calories in versus calories out, or is that oversimplified?

The energy balance concept is technically true at a basic physics level, but it's widely considered oversimplified as practical advice, because it ignores how strongly hormones, genetics, sleep, stress, medications, and metabolic adaptation affect both sides of that equation for different people. Two people eating identical calories can have meaningfully different weight outcomes due to these other factors, which is why "just eat less and move more" doesn't work equally well for everyone even though it's not technically false. References: https://obesitycanada.ca/healthcare-professionals/adult-clinical-practice-guideline/