if I'm backing up to an external drive, is there any reason to not use FAT other then FAT's file size/naming issues? It would be nice to be able to access the content on windows/mac but i'm not sure if there's any reason i have to use ext4 to backup with ubuntu files. This is really only for documents/photos/videos I have on the system.
Generally this can be done but there are some disadvantages:
As an alternative you can install an ext3 driver on Mac/Windows or use NTFS (which doesn't suffer from problems number 1 and 4).
If you want to access the backup drive with Windows too, go for FAT. I have tried using ext3/ext4 in Windows, with some drivers and with some other ext3/4 volume reading softwares in Windows but nothing really works. I got my ext3 volume corrupted once and after that I have decided not to try it again.
And if you dont have any problems with the naming and sizelimit of FAT, there is really no reason for not using FAT on an external drive. If it was an internal drive, which would be accessed frequently, then ext file systems would be preferable because of journalling but for external drive FAT works well.
Also the file permissions and special attributes wont make any sense when accessed in Windows so no use keeping them.