My uneducated answer: if you would try to do a classic 2x20 threshold workout, you could save 500 bucks and know that you aren‘t a 330W guy and your FTP is set too high. It‘s really not hard to find out. Unless ones ego is in the way.
A recent, and much more educated information by Kolie Moore why the Vlamax model has incorrect assumptions:
So why to rely on a model which has proven wrong, is expensive and doesn‘t provide more information than the power duration curve?