Slow cookers are generally recognized to be safe when used unattended - and there are a variety of slow-cooker rib recipes out there. I've made them to great success before - they're fine braising in their own juices , or with a little liquid, such as root beer or Dr. Pepper.
The problem with the oven is the lack of temperature control at low temperatures and the amount of electricity it uses - with a gas oven, carbon monoxide would be a worry leaving it on for 8 or 9 hours at a stretch, as well as steep gas bills. There would be a very low risk of fire, but enough of one where fire safety experts do not recommend leaving it unattended.
