Depends on your definition of 'truly safe'. That might involve wrapping up in cotton wool, never going out and avoiding all sorts of activity. Risk with electrical appliances (as with any form of activity) is all about the probability of a fault occurring times the impact of what might happen if it did go wrong (a fire might occur).
Leaving a slow cooker on all day unattended doesn't change the probability of it going wrong - but being at home minimises the opportunity of a fire spreading - one would hopefully realise before it gets out of hand.
The point is that the probability of such an electrical device going wrong is very very very small. It uses little electrical current - perhaps enough for a 100W of power. Now you know how much heat comes off a 100W normal light bulb - it gets hot but not like an electric fire (of maybe 3000W of power). The chance of such a device catching fire and setting fire to something else is really slim.
So it's the same order of magnitude of danger of leaving a fridge on all day. You do that, don't you?