I think it's like one of two criteria:
1. Damaging to your body and highly addictive. This is most opioids, arguably tobacco (which is kind of grandfathered into being legal), meth, cocaine etc.
2. Can cause significant, unpredictable shifts in behavior in otherwise stable people, to the point of endangering themselves or others. This is (also) cocaine and meth, arguably alcohol (also grandfathered in), ecstasy, and why LSD is so highly scheduled. There's probably a world in which there're responsible, safe ways to experiment with LSD, but it's not something you necessarily want your given random guy on the street to be taking, lest he walk or drive into traffic or start swinging a 2x4 at people.
Ease of addiction combined with difficulty of quitting.
If that is the criteria, it is clearly wrong to include LSD in that bucket.
Oh, for sure. "Hard drugs" classification makes sense for cocaine, heroin, fentanyl, but not for psychedelics.
degree of effect. marijuana and ecstacy are "soft" because, in common recreational doses, they have less radical effects on one's personality and coherence. heroin, acid, and psilocybin all have more significant effects, microdoses aside.
I would argue that MDMA in common recreational doses has about the same level of radical effects as psilocybin. Different effects, but similar "level" or intensity of effect. And MDMA is much more dangerous for your body.
Opioids including heroin have moderate effects at moderate doses, much like marijuana or alcohol.