Is exFAT stable enough in Ubuntu 14.04?
Well it is in use fairly widely and I myself have not seen users with problems where they are loosing data due to bugs in the exfat/fuse implementation.
With all filesystems that are not native to Linux: be careful when using them and always keep a copy of Windows on your system (even when not using it: it is safer to shrink the Windows system to a bare minimal and edit grub config to skip asking for what OS to start then to remove it completely. There will come a time where the partition is un-mountable from Linux and you will need a Windows tool to fix it).
Just for 1 reason only I would stick with NTFS: exfat is only available as a personal archive and ntfs is in the repositories. So it seems Canonical does not deem it good enough to include in the repositories yet (yet since it is included in Debian so I would expect it to be included too in 14.10)