It's also a victim of the Heisenberg Uncertainty Principal. You can't measure your 60' power without impacting your 60' power. It would require a taper, where you lose CTL, and a recovery, where you again lose CTL. And all of this would impact any races that were in the two surrounding weeks. When it's all over, you FTP won't be the same as what your test result shows.

On the other hand, if you don't taper properly, then you're coming in lower than your real threshold.

What's really important is setting your training zones and tracking changes. Many of the other deadly sins are perfectly valid for this use. If you want to go throwing around an e-wang, that might be a different story, but for training, something like MAP or Monod are really quite good, and pretty consistent in my experience.