Sysupgrade - dangerous secrets

Using sysupgrade I noticed some "peculiarities" and possible changes of functionality during different versions of openwrt.
Actully, sysupgrade silently sometimes does not install a new version of custom built image, BUT updates certain files. I.e.
sysupgrade -v -n -f /tmp/newfiles.tar.gz /tmp/fw.bin
updates all files, contained in newfiles.tar.gz, but does NOT flash fw.bin. Causing a real PITA, as a reboot is done, too, causing an inconsistent system.
I suspect, not doing the flash is because of active processes/open files on mounted block-storage/active swap or something.
I think, in the past sysupgrade at least refused to do anything, in case of problems.

Is there any uptodate doc about sysupgrade internals, especially regarding error conditions ?

That sounds like a bug and not intended behaviour. The intended outcome should be mentioned in the —help output