I am doing cardio regularly. Bike, swimming, running. According to my docs, my lungs, heart etc. are all great. The app shows two lines and three areas. One above which seems to indicate I can do lots of sports and workout hard, pushing myself, the middle area and the lower one which seems to tell: rest, or do less or take it easy etc.
According to this app I should never ever do more than walking around my car. I do my sports, I sleep, I have my rest cycles, all approved by track and field coaches and running coaches and docs. But this app not once over several weeks showed me more than a 4.6, which is in the lower end of the middle area meaning, keep the intensity easier. No matter how much rest I took, how hard I trained or how bad or well I slept.
I use the watch for all my workouts and wear it as much as possible.