If I put my cynic's hat on, they're especially terrifying to drug companies. If you take SSRI's and they work, you'll take SSRI's every day for the rest of your life. If you take LSD or MDMA or ketamine and it works, you're done after one dose.

Anecdotally, people who regularly took LSD seemed to get increasingly weird compared to those who only took it once or a small number of times (c.f. http://slatestarcodex.com/2016/04/28/why-were-early-psychede...), so if anything, a persistent effect might turn into a compounding effect if you don't restrict yourself to a single use.

Also, there are cases (cf http://slatestarcodex.com/2018/06/06/hppd-and-the-specter-of...) of psychedelic drug users suffering persistent bad effects.

The interrelation between physiological and psychological mechanisms is still largely unknown, but what is known is that even normal human experience can sometimes have a persistent one-time psychological effect. The possibility of harnessing such an effect for therapeutic purposes is concerning, perhaps, but no more than any of the other mysterious mucking about we tend to do.