It's broken thinking to assume something is safe on the basis of a lack of proof otherwise. We have a pretty substantial body of research exonerating cannabis of a lot of the harms other drugs of abuse can bring. Literally no-one ever has died of a cannabis overdose, that we know of. We know it's safe that way.

There is of course a risk with any mind-altering substance that it might trigger a latent or occult mental illness. Those odds go up, sometimes significantly, with more powerful substances, and even more so when the user has any genetic predisposition towards those conditions. There's also a risk of having a difficult or traumatic experience, which can likewise increase with dose or different substances. That said, those of us who work with these substances intentionally often find that those experiences can be where the really transformative stuff happens.