First up, for those running UBIFS on raw flash memory, there is now OverlayFS support. OverlayFS, as a reminder, provides a union mount for other file-systems. O_TMPFILE, RENAME_WHITEOUT/EXCHANGE are now supported by UBIFS for handling OverlayFS.
The UBI code in Linux 4.9 also has went through some code refactoring in prepping for MLC NAND support. For background information on MLC NAND support for UBI/UBIFS, see this mailing list thread for some of the earlier discussion.
More details on the UBI/UBIFS changes for Linux 4.9 via this pull request.