I triend to use sysupgrade on a cluster where /home is mounted over NFS.
It didn't work. I think it might be possible to write a hints file to
make it work, but it is brittle.

In the mean time, the following patch appears to stop it right in its
track before wasting time downloading files to NFS.

Index: sysupgrade.sh
===================================================================
RCS file: /build/data/openbsd/cvs/src/usr.sbin/sysupgrade/sysupgrade.sh,v
diff -u -p -r1.52 sysupgrade.sh
--- sysupgrade.sh	19 Jun 2024 05:22:33 -0000	1.52
+++ sysupgrade.sh	10 Jun 2025 16:54:11 -0000
@@ -132,6 +132,11 @@ fi
 
 install -d -o 0 -g 0 -m 0755 ${SETSDIR}
 cd ${SETSDIR}
+case `df ${SETSDIR}` in
+*:*) 
+	echo "Error: ${SETSDIR} on NFS is currently not supported"
+	exit 1;;
+esac
 
 echo "Fetching from ${URL}"
 unpriv -f SHA256.sig ftp -N sysupgrade -Vmo SHA256.sig ${URL}SHA256.sig