A potential problem with testing based on power or RPE (feel) is the mental component. Sometimes athletes will undershoot their physiologic capacity because of mental weakness. If that is suspected, a good MLSS test can help separate legs from brain. If you can identify a problem you can take a shot at addressing it.
This works in reverse as well. If an athlete is convinced his/her FTP is 350, but their MLSS is 325, then they have actionable data. As an example, this type of thing is relevant if pacing a long TT by power.
I don’t mean to push MLSS testing as that has its own issues. But in some circumstances quantitative data can be useful. Mostly, be aware what you feel may not be what you can do. And RPE is not always telling you the truth.
Carry on!!