There are many reasons why training with power is more effective than heart rate, first of all being the influences other Maximum Power XL factors have on your pulse. I currently take daily medication which gives me an elevated pulse, so if you were to measure my resting heart rate you would have to do it in the evening right before I take the next daily dose of medication. This will already give an influenced result as, what most people will tell you, if you take your pulse in the evening you are taking into account a days' worth of work, exercise and so on. However, my pulse is even faster still in the morning, meaning another influenced result.
https://goldencondor.org/maximum-power-xl/