Link /etc/systemd/system/syslog.service still points
to /lib/systemd/system/rsyslog.service which does not exist after
installation syslog-ng.
The link should be processed in the "install" section
of /var/lib/dpkg/info/syslog-ng-core.preinst script too. Maybe it is the
rsyslog package which should take care of.

Peter