Does this work for you? Here it causes this:

systemd[1]: /usr/lib/systemd/system/boinc-client.service:14: ReadWritePaths= path is not absolute, ignoring: $BOINC_BOINC_DIR
systemd[1]: /usr/lib/systemd/system/boinc-client.service:15: WorkingDirectory= path is not absolute: $BOINC_BOINC_DIR
systemd[1]: boinc-client.service: Unit configuration has fatal error, unit will not be started.

While /etc/sysconfig/boinc-client has BOINC_BOINC_DIR="/var/lib/boinc", the error indicates that systemd doesn't do any expansion. See also https://stackoverflow.com/questions/37864999/referencing-other-environment-variables-in-systemd.

I don't think this will work. To solve boo#1227092, we can either hardcode the working directory, or replace /usr/bin/boinccmd --quit by something like /bin/sh -c "cd $BOIND_BOINC_DIR; /usr/bin/boinccmd --quit", because the shell will expand the environment variable.

Or we get rid of this ancient /etc/sysconfig/boinc-client stuff and let users override the data directory by defining a drop-in configuration file that overrides WorkingDirectory=. I think that without --dir, the BOINC client will simply use the working directory.