to be honest, I haven’t updated the empty accession numbers, except for a few old records that I still cite, where I had to refind the records in pubmed and copy the accession or doi, so it could update record in the original library – I still wish endnote could combine duplicates, but then I would like the option to let me define WHICH record number would be retained!
I personally haven’t had any issues and acc numbers appear to be unique, however, 99% of the time, I use my own computer or a computer with access to my library on the network drive (and yes, I know the developers recommend against a networked drive used for the library, but after 19+ years – I am absolutely an “old dog”. ). I only use the cloud version of the library when I am traveling with my laptop and am usually not doing that much heavy editing of manuscripts. Mostly retrieving PDFs for review, etc. Even then, I try to remember to copy my library to the laptop before leaving, so that I can work offline.