Fwiw, I measured with a tape measure when I calibrated the 50 Art, and as far as I could tell, the measured distance to the sensor matched the distance scale spot on.
Lens focus distance scales are normally pretty accurate, their dof scales are normally next to useless, especially with modern short throw focus AF lenses. But the point about a lens being designed for ff and used on crop cameras making the aperture/dof numbers completely wrong is an interesting one and maybe a reason most lenses don't have them now.
Too often we lose sight of the fact that photography is about capturing light, if we have the ability to take control of that light then we grow exponentially as photographers. More often than not the image is not about lens speed, sensor size, DR, MP's or AF, it is about the light.