Sorry, but this is a terribly wrong myth. Each lens is designed for optimal convergence of light at a specific flange distance (register distance). At this distance maximum resolution (and minimum aberrations) is achieved. Some lenses are more prone to aberrations/degradation when this optimal distance is going to be altered, some are more tolerant. The more tolerant ones are those that are most suitable for being used with bellows or extension tubes.
One of the best macro lenses ever made, the Leica Apo-Macro-Elmarit-R 100mm f2.8, is relatively prone to flange distance changes, and Leica does not recommend using it with any extension! Same goes with the Fujifilm XF 60mm f2.4 Macro. I had/have both and can confirm that from experience. The Leica performs much better with Leica’s Apo 2x teleconverter than with any extension (the converter doubles the reproduction ratio as it retains minimum focus distance). The Fujifilm XF 60 I tried with the Fujifilm extension tubes – and gave up.
As for the main topic here, the XF 90 with Raynox 250 is a surprisingly nice performing combo indeed.