"Are they safe though?"
They should be.
I think it's always best to give a new slow cooker a couple of trial runs while you are around so that you can make sure that the thermostat is working ok.
On the slow setting the cooker should cycle around 95c (- / + a couple of degrees)
The 'auto' setting (if it has one) should take the temperature up to virtually boiling point and then settle back to 'slow' cook after a set period.
The 'high' setting' should hold the contents at near boiling point - 96 / 100c.
The most likely problem to arise is a faulty thermostat - there is actually very little in a slow cooker to go wrong.
If it is running at too high a temperature on the slow setting, the food can become mushy and even meat may fall apart and break up. If the temperature is too low, then the cooking time may be too long.
If a thermometer is not to hand, then after an hour or so (with lid on) the liquid content should just show some slight signs of movement due to the heat. If it's a brisk simmer, then the temperature is too high. If there is no sign of activity at all, it's probably too cool.
Some slow cookers have vented lids, others don't. If the lid is not vented, then it may rattle every now and then as the steam pushes out, an unvented lid that rattles continuously is a sign that the cooker is running too hot.
Lifting the lid too often on a slow cooker will set the cooking times back considerably - some of the lower powered ones are no more than a 150watt - which is what the old type large light bulbs used to run at.
Obvious safety factors are to stand it on a hard surface with plenty of space around it.
Always make sure the right value fuse is fitted in the plug - a low powered slow-cooker will be fine on a 3 amp fuse.
