Is it ok to leave a slow cooker unattended?
That's exactly what they are designed for!

In general their electrical construction, and possible failure modes fully support being left on unattended

They should pose no more fire risk than any other electrical kitchen device being left on at the wall e.g. an automatic toaster or kettle

Some slow cookers have automatic fuses that blow if the pot runs dry, but most seem to keep going with no ill effect. There is no reason for a slow cooker to run dry unless you leave the lid off for over a day!