Yes, they are compatible and you can see the stars in Adobe's programs as well. The only problem is Adobe's bad metadata support, i.e. its incosistency in reading embedded XMP over sidecar files XMP. Complain to Adobe
You must set up Photomechanic with these options:
"Add Tag, Color Class, and Rating to IPTC/XMP" : FOR ALL PHOTOS (Files tab in prefs)
and possibly also the correct order in IPTC/XMP prefs tab (search the forum for Adobe compatible setting, I don't remember it at the moment).
It works well, the stars are perfectly visible in Bridge/...