<< I think your analogy to the marathon is right on target. PE is too unreliable in long distance racing as the appropriate pace early on usually feels too easy.
actually what you are saying is that your ability to monitor PE isn’t good (no disrespect intended). If you do the workouts and know that you should be able to average 7 minute miles for a marathon for example, then you also know that the first 10km at 7mpm pace is going to feel relatively easy. In this example all you need is a stopwatch and you should be able to monitor PE pretty accurately. On the bike it becomes a bit more complex with wind etc. Personally if I was going to have one or the other I would prefer to use a heart rate monitor because it does take external enviromental factors into consideration while a power meter alone is just a measure of your power output. If something is off such as dehydration, onset of hyponutremia and so on, wattage will be affected but you may have no idea why your numbers are off. With a heart rate monitor if you know what you are looking for and how to decipher the numbers, you can read a lot more into it then with a power meter. If I were to use a power meter, I would always use it in conjunction with a heart rate monitor so that I would know when to make adjustments to my power numbers in a race based on the heart rate feedback.