TL;DR: Keep your easy days easy so your hard days can be hard.
It depends on the goal of the workout, which is kind of what I was driving at. Base phase, you’re usually establishing the aerobic fitness for whatever event you’re doing. (Keep in mind I’m generally well-schooled in longer stuff, triathlons specifically… may be a little different for crit or XCO or enduro types). When relatively untrained, your HR will track poorly with power over the longer rides (aerobic decoupling). Once your aerobic fitness is up to snuff, then you’ll see far less of an increase in HR relative to stable power over longer and longer periods, and your HR at a specific power will probably go down as you get fitter. In fact, some coaches use aerobic decoupling as one primary metric to determine when it’s time to move from aerobic base building into training more advanced limiters like muscular (strength) endurance or anaerobic endurance, while just maintaining aerobic base.
If you use HR later in the season, when you’re just trying to maintain (rather than establish) aerobic fitness, and you’re trying to improve muscular endurance, you run the risk of training the muscles with a harder stimulus than desired when your intent is to maintain aerobic fitness and give the legs a relative rest. This could be training in HR zone 2, but in the tempo or sweet spot zones of power, which can lead to the old “easy days not easy enough” problem, and cause issues with respect to the quality of your programmed hard days.
To avoid this in build, when you want to come into hard workouts relatively fresh so they can be hard, many coaches will prescribe zone 1/2 power rather than HR, because you can maintain aerobic fitness in a very broad range of bpm. If you go into all your “easy” workouts in a build or peak period trying to ride at the top end of zone 2 HR, odds are you are working too hard on your “easy” days, and you won’t get as much out of your hard days.
Programming aerobic rides by HR in base and power in build is my preferred method. Doesn’t mean that’s right for everyone, but it’s worked well for me.