High stress (mental, physical, lack of sleep, lack of food) will never be good. If you are losing weight in a high stress environment of low calories and high activity level (like boot camp) you will get lean....and then once refeeding happens down the road the bodyweight will self-regulate itself back up storing more fat (stubborn). Hence all the yo-yo crash dieters.
If you already have stubborn fat, increasing stress more is not going to help and only create more and more of it. Hence why IF can work, but not with someone that has fitness ADD working out 7x/wk at high intensity while eating 800cal/day and sleeping 4 hours/night.