Calories in / calories out works perfectly and predictably for fat loss as long as you’re measuring correctly and most people don’t. I used to follow it religiously in the run up to fights in order to make weight.

I would look at my weight, look at the weight I need to be 8 weeks later, figure the calorie hole I needed and follow it incredibly closely.

It worked every single time without fail. I always stepped into the ring lean as fuck and having lost zero muscle because I was training twice a day six days a week and keeping protein intake high. I allowed one maintenance calorie day per week on my recovery day.

There are other things that account for weight loss (e.g. loss of sugar from muscles and the associated water loss that comes with that) if you’re using keto but most people are after fat loss more than weight loss.