As long as the cooker is in basic good order, then you really have no worries whatsoever from a safety perspective.
They are very cheap too - they use about the same energy as a single lightbulb, you can buy the cheapest cuts of meat and cook them really long and slow (Lamb shoulder, beef shin or skirt, pork belly, oxtail) and they still come out delicious because they have had time to render out the fat, break down the sinew and soak up all the flavour of whatever you cook them in.
They are economical, convenient (so long as you can make 15 minutes to throw whatever in the pot before you go to work) and convenient - get home, lovely hot food waiting for you to eat at your leisure.
And don't just think stew - you can cook porridge overnight and wake up to perfect brekky, you can cook cakes, whole chickens, you can use them as a bain-marie by placing an earthenware dish with a tight fitting lid on an upturned saucer and filling around it with water so you can make pate, terrines, cheesecake, confectionery etc etc (try thickening up a tin of condensed milk for hours - nom nom nom)
You can even throw in frozen meat, water and a stock cube and come home to a feast.
Not only that they are cheap as chips however its worth spending a little extra for one with a count down timer that switches to a keep warm setting then you never have to worry. I have
this one.
Can you tell I love my slow cooker?