As far as whether it's safe, that's like asking if Gasoline is safe. If it's used in a proper manner, yeah it works really well, but if not...well then your ass could be on fire.
In moderation they are safe.
Now some are safer than others...like Caffeine, a stim, is not nearly as hard on your body as ephedrine, and ephedrine isn't nearly as bad as DNP...so you just gotta do your research.