(me:) Are there still known problems with keywords written in PM+ not always being recognized by Capture One?
This I do not know.
I've spent a while today with Photo Mechanic+ and Capture One, updating keywords and other metadata first in the one program and then the other—and sometimes editing the XMP files manually outside either program to see what would happen.
So far Capture One—version 21—hasn't ignored or tried to delete any keywords that were added within Photo Mechanic. That was a problem in the past.
Both Photo Mechanic and Capture One put keywords within the
<dc:subject> element. Capture One adds—
duplicates—the keywords also within a
<lightroom:hierarchicalSubject> element when you tell it to synch metadata. Since the program accepts the <dc:subject> element's contents when it's the only one containing keywords, no idea why they'd also create <lightroom:hierarchicalSubject> in the XMP file. Doesn't seem necessary.
(Not to mention, that's the name of their major competitor. :-)