I just searched some hours because rauc wount update my rootfs. The message was:

Installation error: Unsupported image /run/rauc/bundle/MyRootfs.tar.gz

I found out that I had some trailing whitespaces in the system.conf at type=ext4

Because of that the g_pattern_match_simpel function in get_update_handler (update_handler.c) didn't match.

Maybe it is possible to remove trailing whitespaces before calling g_pattern_match_simple function in future.