Rucking, like distance running, needs calories to burn to get better workout results. Yeah, you burn calories on an empty stomach but performance will suffer. The proverbial car trying to perform on no gas. The con is you run out of fuel. If you do it, stay hydrated. A pro is you burn calories faster when you do eat.
Intermittent fasting is good for some things and not for others. Here is how endurance athletes can decide whether it is right for their performance goals.
trainright.com
Fasting is defined as the abstinence from consuming food and/or beverages for different periods of time. Both traditional and modern healthcare systems recommend fasting as a therapeutic intervention for the management of several chronic, non-infectious ...
www.ncbi.nlm.nih.gov
RE: fasting, there is no evidence it works better than any other calorie restrictive 'diet' (
https://www.inverse.com/mind-body/intermittent-fasting-no-better-than-dieting), but if you have the discipline to stick with it and it works for you, then stick with it!