One thing we found was that the way the out-of-the-box taxonomy connector is implemented is dangerous, because it operates on TCM IDs from Tridion Sites to represent keyword/tag custom metadata values in Tridion Docs. This is dangerous because when migrating content across DTAP environments, the IDs will differ, causing invalid metadata values in higher environments.

One potentially better solution would be to instead use the WebDAV URLs or even the keyword values (which will be unique within a given category) from Sites to represent the metadata values in Docs. Better yet would be to allow the option to choose between using the TCM IDs, WebDAV URLs, keyword value or keyword key field.