Heheheh. As I’ve already mentioned somewhere else, I have been doing this metadata stuff for nearly four years now, as the heart of my own app. And I have read countless documents on how to, how not to, etc, in order to try and create an app that writes metadata that is truly compatible to everything else out there.
But then, during a chat with @platypus, he mentioned that my app couldn’t yet import a user’s existing keyword dictionary. Oh boy! Was I in for a surprise?
He pointed me towards some readily available dictionaries that were available for downloading and I set about “reverse engineering” one of them so that I could write the code to import it to my app. Which is when I discovered the “delights” of categories and synonyms. I show an extract from one of these dictionaries in this post
Maybe it gives you some idea why it would be better to concentrate on getting “ordinary” keywords right first 
Unfortunately, this is a result of other app, as well as PL, doing weird stuff to make an “acceptable” UI for keyword entry and display.
The greatest unifying and facilitating factor in managing what gets shown versus what gets stored would be if everyone adhered too the xmp-mwg-kw:hierarchicalkeywords tag instead of “making do” with the, far less expressive, lr:hierarchicalSubject tag that Adobe seem to have enforced as standard. The addition of the Applied tag would make life so much easier.
Unfortunately, not having a Mac, you never got the chance to try out my app.
My vote, not surprisingly, goes for the PL5.1 variant - I don’t know why DxO ever reverted from it.