Hmmm, savedvariables is account specific so if I run 2 accounts on the same computer and run census on both there is definately an issue with different submissions overwriting each other. It's not a major issue but it does cause problems in two scenarios:
Anyone using 2 accounts to run census on both sides of multiple PvP realms will generally want to upload and purge fairly often.
More common is simply playing on your two accounts and then before you go to sleep you log off the game, if you then have census data from several accounts you will either have to wait a little under an hour between submissions or wait until the next day to upload 2nd account (and third...).
Not a major thing but there is an issue, something to keep in mind though is that you actually save your data as soon as you log out a character, not when you close the game... if you run census on multiple realms or simply do a lot of censusing (and have a machine with a decent amount of RAM) you might consider alt tabbing out to upload. There is a good reason why the limit is there, intentionally or unintentionally uploading essentially the same data will slow submissions and we all want the data to get updated as soon as possible once we upload
