Since I've never used backups or upgraded,
but if you update, do you have to reset everything?
Is the light backup not working properly?
Does upgrading from one firmware to another carry over the entire configuration, or does it have to be redone?
Is the backup reliable even if you revert? Does it restore everything?
Upgrades by versions like 4.x to 4.y are not an issue, config will be upgraded as well.
Upgrades by major versions 3.x to 4.x require an reset.
Backups are a difficult topic. The luci backup mechanism is pretty dumb and just dumps all config files - no matter if they are compatible with other firmware versions. Mostly it will work fine as long as the main firmware did not introduce or removed additional features.
It is also usefull to know, that the backup archive from luci is just a 7zip file, you can manually compare configuration and you can also delete everything except for /etc/config and then reupload the backup in luci this works and gives you a more cherry picked lighter version.
Most of the contents on /etc/config are saved but sometimes incompatible if GL-iNet changes something, normally it shouldn't break by OpenWrt design, it can break however when you move between the vendor sdk and Vanilla OpenWrt or OP24 flavour because the driver references are not matching in the wifi config.
Other configs are normally discarded outside the /etc/config avahi and such are discarded, you could however preserve them by adding them to the file in /etc/sysupgrade.conf, the startup logic for custom scripts is in /etc/rc.local and likely also not preserved incase you modified it to your hearts content.
Normally when I choose the cherry pick approach I just compare configs in winscp with the ones from my backup file to have a understanding what is going on.
Between 4.8.x and 4.9.x it was a reason for me because they changed the vpn software with a note about the killswitch, but I did not found a big issue, but there was with port forwarding disappearing from gl ui but not luci.