I have had bad trips on everything but the worst was on DMT, oddly enough. The best worst trip was on mushrooms. It just made me aware of what an asshole I was being. Mushrooms are a good choice if you want to experiment, because they are over quick. No matter how bad it gets, it'll be over in a couple of hours. Acid and DMT, on the other hand... if they go bad on you, you're in for an eight hour ride, at least. Maybe longer.
If I took acid while depressed and with no supervision, I would almost certainly kill myself, but probably botch it for lack of competence. As we used to say back in the day, "Acid makes you just like you are now, only more so."
I would be willing to try mushrooms for depression or other mental illnesses, but only with good psychiatric supervision — professional or otherwise.
Also? DMT gave me one of the BEST trips of my life, which really did make a change after I came down. What I learned on it was not an illusion. So I can very well see that, in the right hands, with the right psychiatrist, it could be effective in helping someone. Use it on your own, however, with a mental illness, and you are basically playing Russian roulette.