How Apple treats Aperture adjustments changes with each release (I did not know this until yesterday – I assumed they had some conversion code and left it at that). In Mojave, Aperture edits are considered external edits. I do not know if Aperture adjustment data is stored in the library for future use, or if they just store a placeholder that says “nothing to see here – external editor was used”
I ran two tests today. One with half-sized Aperture previews and full-size previews. With half-sized previews, the images came through upgrade with NOTHING at all. The images were treated as unadjusted. With full-size previews, the JPEG appears. Attention: people who are upgrading their Aperture libraries in Catalina – make sure you have your previews set to “No Limit” [aka full-size] and ALSO rebuild all of your previews (changing the preview preference in Aperture affects future previews, not existing ones).
(If you already upgraded in an earlier release, you have already have full-size previews)
While this does serve as a warning for people, any time you have data in an application, you are at risk of losing it. This is true of RAW Power as well – if the app stops working for some reason, then you would not have access to the adjustment data either. Apps can conceivably document their adjustment formats, but someone still needs write a very good converter.