Im using everything in running, save power. Not reliable.
On a normal day where i 1. Dont feel fatigued (RPE) 2. HR is trending normally depending on am/pm 3. i can almost tell you within .5 mmol where my lactate is based on HR achieved, with a secondary check to my subjective feeling RPE.
Many, many times RPE is not in sync with what is going on metabolically.
HR is almost never not in sync with what is going on metabolically. When it is then it's time to start working backwards to analyze CTL and whether or not that has produced excess fatigue.
As Sirpoc said, in cycling, Power is in the trinity of metrics to use. Then i know if i have 2 of 3 (HR, Power, Lactate) you are assured to be dialed in.