Started with HR in 2016 and it served me well. Quickly figured out that I blew up when spending more than 3-5 minutes at 165bpm. Then figured I could hold something like 155-160bpm for 20 minutes, then 30 minutes, then 40 minutes, and eventually worked my way up to 50 minutes. Also figured out I could do long 1-3 hour climbs in the mountains at around 135-145bpm, and somewhat easily finish centuries at that heart rate.
Added power I found power zones correlated really well with those heart rate zones (159-161 bpm is threshold heart rate, steady endurance/zone2 rides are 135-142 bpm, tempo/SS is 144-152 bpm, etc). However training in flatland, into headwinds, is a lot like doing long steady climbs where its easy to hold steady power and HR. No real surprises after adding power. Power simply made it easy to do shorter high power intervals. Before that all of my short power intervals were all-out because you can’t really train by HR on short intervals.