A database update with ‘global -u’ doesn't seem to find new files, i.e. files that have been added to the project since creation of the database. That feature would be useful. Not sure about the performance overhead since all directories will have to be scanned again including application of rules, etc.
While a full gtags run isn't too painful in my case (thanks to many skip rules), I just got bitten by the fact that while ‘global -u’ updates the existing project DB, ‘gtags’ creates a new DB in the current dir.