In prod, the relationship editor is (sensibly) limited to only batch-create new works for recordings that do not already have a work linked. That means you can easily add all the missing works once you've linked all existing ones by just selecting all recordings and batch-creating the rest. The beta relationship editor will happily create a new work even if one is linked, which is almost never going to be a good idea (and makes it a lot more annoying to use the feature since you need to specifically select only the recordings that need a work).

If there's a strong enough use case for adding new works even when one exists (which I have needed to do a couple times, so I wouldn't be against it being possible) then it could be done with a checkbox "also add works if one is already linked" under the batch-add link or something?