Not sure your comparison is correct. Power meters are a direct training method that is used to structure your entire training season and critical in training percision. Also very useful during most races and invaluable in post race analysis. HR monitors and the data derived over time can give you an indication of your rest, recovery, illness, over-training, stress, etc. Also power meters and HR monitors are readily available, useable and have countless research surrounding their use.
Metabolic testing might be the next best thing, but let’s not exaggerate it’s importance and diminish well established fitness tools.