>> For example OP track his calorie intake, it is a lot of work, and it is not really usefull at the end. It is unlikely to correlate with any other measures he is doing (apart from weight).

Tracking calories is very very simple if you use an app like MyFitnessPal. For most foods you can scan the barcode or you can just enter the calorie data from the label.

Secondly, isn't weight a pretty important measure to track? It can impact a lot of things and therefore if you see you start to have too many days in a week that are above your calorie goal you can catch any gain before noticing it on the scales.