Been here a lot with proper structured training lately.
Training with power and HR then utilising a PMC is very worthwhile if you don't. RPE is a waste of time for gauging where you are at and how you should be performing as I've seen me feel awful before a session and then on the bike I perform amazingly, then other days feel buzzing for my session only to get on the bike and feel totally flat.
Acute and Chronic fatigue is a hard one to master but worthwhile trying. Along with diet, previous few days sleep, stuff going on in your life off the bike etc. Heart Rate is a great first brief and quick look analysis (sometimes). If it is high in the morning or higher than normal when you are warming up this is a sign that you are under heavier fatigue and rightfully won't perform at your best, BUT, I have recently had a bout of low HR and being unable to raise it and was feeling fla/underperforming, so even that can go both ways.
Basically, power alone, HR alone or RPE is not the answer to gauging your body/performance/fatigue. Combinbing all, using a PMC to track and forecast and pulling it all together is as best as you are going to get to nailling it, but even then it won't be 100%
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The joys of the human body!