I run various Pi computers around the house, one of which is a Pi Zero W. This has had Daedalus running on it for a couple of years without worry, but I decided to move it on to Excalibur. I checked the usrmerge and that seemed to be already done - so I changed apt's sources.list to point to Excalibur, did an update and then a full-upgrade.

The full-upgrade crashed with an error message about mkinitramfs-tools and mentioning hooks to kmod.

The possible bug is within  /usr/share/initramfs-tools/hooks/kmod. Towards the bottom of the short file it gives two paths:-
/usr/lib/*-linux-gnu/liblzma.so.5
/usr/lib/*-linux-gnu/libzstd.so.1

On a PC, the * would match "x86_64" or  "i386" and on an Pi it would match "arm". But on my installation, the directory it is trying to copy from is /usr/lib/arm-linux-gnueabi/libwhatever. The absence of the eabi after gnu prevents the system finding the files and triggers an error. I edited /usr/share/initramfs-tools/hooks/kmod to include the eabi and ran dpkg --configure mkinitramfs-tools and it ran perfectly.

Whether this is a bug or a common problem or specific to my Pi - I do not know.