I thought that I would mention that some people are unpleasantly surprised when they find their expanded overlay reset to default size and all their data there gone after an upgrade.
It is my understanding that the the upgrade process is pretty much the same as an installation except that config files and other files set to be kept, get copied to /tmp and then written back to their locations after the image is written to flash/disk. The upgrade copies the downloaded system image to the raw media overwriting the partition table, partitions, filesystems etc.
I was expecting it and it proved true on my nanopi r4s.
The nice thing about asu is that the packages are preserved on /rom and related configs in /etc/config/ are preserved on the upgrade. I even set it so files on /root/ and sub-directories were preserved.
Enough space on /tmp needs to be available for the image and config files etc. so that is a potential limitation.