I suppose you shouldn't cook raw meat or raw poultry in it since it cooks on low temperature
I always brown the meat or poultry before I put it in the slow cooker, as that locks in the flavour and it just looks better. If your cooking things on a low heat then that can take 8 hours or more, If I do a casserole on low temperature it would be cooking for 10 or 12 hours, so raw meat or poultry isn't really a problem. But as I say, it looks better if you have browned it off first.
A lot of slow cookers now have an automatic setting (ours doesn't) which cooks things on high temperature to start with then reduces the heat once it's cooked (4 or 5 hours) to keep it warm until you are ready to eat it.