Comment 21 for bug 1546641

Looks to be a race condition where the session (and so unity-settings-daemon) is started before systemd has finished starting the upower.service unit.
u-s-d tries to query the state of power and segfaults when upowerd hasn't finished initialising to be able respond.

From syslog:
17:14:31 kernel: unity-settings-[1722]: segfault at 0 ip 00007fe3799805a9 sp 00007ffec6513af0 error 4 in libupower-glib.so.3.0.1[7fe37996f000+25000]
<snip>
17:14:50 dbus-daemon[460]: [system] Successfully activated service 'org.freedesktop.UPower'

Confirmed this by doing the following:
1. Disabling lightdm from autostarting: systemctl disable lightdm

2. Reboot and wait until systemd starts upower.service

3. Start lightdm: systemctl start lightdm

4. u-s-d starts without issue and so default settings and themes are applied, media keys work, etc.