SD may cause system re-boot to take a very long time or cause F-Secure\SD to malfunction - or all three to happen.
Worst case scenario is if SD malfunctions while exiting Shadow Mode, malware is still on system when reboot... unlikely to happen, but theoretically possible if some other soft causes SD malfunction.
In my experience SD is reliable, but it doesn't work well with all AVs on my specific system - eg F-Secure, Webroot, etc. Causes system and/or other softs to misbehave.
Who is at fault - the AV or SD ? The answer is neither - the softs just haven't been tweaked by the devs to be compatible. It is difficult to make a soft compatible with everything out here in user land.