I posted a similar question awhile back:
We are relatively new to DITA. Our writers have been adding cross-book links to their content (which is calling our bluff on finding a PDF/OLH solution to publish them properly!). As a result, we decided to use keyref cross-references for all topic references, both within-book and cross-book.
The easiest part has been the creation of the topic cross-references in Oxygen XML Author. Notably,
The hardest part has been getting writers to remember to create key values for new topics. If they don't, there's nothing to reference. I've filed an Oxygen enhancement request to automatically populate the key values of new topics, similar to how the topic filename and @id values are pre-set.
I wish there were a "Paste as link (keyref/href)" that would prefer keyrefs but fall back to hrefs. Then I would keep only that in the right-click context menu to guide writers to the best practice by construction.