i look at the more specific yin and yang of drugs is....'experiential drugs' and 'effect drugs'.
psychedelics being drugs which induce an 'experience', being unpredictable as to the specifics of effect as well as the amount of comfort and/or contentment which will come about through their use.
whereas 'effect' drugs imo, are drugs taken because of a desirable sensation which occurs from the intake of that drug. these drugs, you can tell 'whats gonna happen' from doing them, i class alcohol and pills as being the primary examples.
in this spectrum mdma and disassociatives are somewhere in the middle, depending on the use and function each individual is providing. weed too, i feel is somewhere in the middle, often taken as a 'typical stone' to many and disregarded in terms of experiential variation. but for myself, and many others i know, weeds can be unpredictable and provide trance experiences not unlike the traditional psychedelics.
i lost interest in 'effect drugs' ever since i experienced a couple psychedelic breakthroughs. and i do consider these effect drugs 'hard' drugs generally speaking.
but the other distinction i draw, and i'm a unique case, as i'm a tripper but i am also a raw-food vegan and very specific about what i allow in my body. this being said, the other yin and yang distinction between substances is toxic vs nontoxic. and i choose to abstain completely from alcohol because i don't like 'even a little' bit of neutoxicity, hippocampus degeneration, dendrite problems (whatever crazy shit it does up there). i also abstain from mdma and all pharmaceuticals and Also the disassociatives. i may be wrong, its been a bit since i've read about the physical effects of disassociatives, but i remember them being hard on the liver...not to the extent of liver failure from a few experiences, but things which have a 'heavy' 'low dose of something toxic' effect on the body are things which i choose to not do.
i meditate daily, and can feel the difference between a body at 100% and a body at 99%, and seeing as though i have infinite fun with the substances which i allow into my body, and definitely don't feel like i'm 'missing out' by not doing these others, i personally...choose 100% :D however, most people, who don't meditate daily and are not actively working to detoxify their bodies and refine their diets, may not care about the difference or be near any conscious awareness of these 'subtle differences'.
anyways, i lost intererest in 'effect drugs', and then have slowly added things to the list which 'are heavy in the body' or 'hard to process' which are to be avoided for myself as well. i have a very 'extreme' view on what i personally allow 'clean enough' to enter my body in both diet and drug use. but for me, 'hard' is anything which is used for personal bliss, or something which I KNOW is not good for my body.
i valued my dxm experiences quite a bit, and i grew mentally and spiritually through them. but, i don't want to tax my liver from its consumption. and honestly i don't know if i'd do it again anyways even if it was pure, i don't really ressonate with it anymore, but thats besides the point. also the whole 'olneys lesions' thing seemed kinda freaky. i know its disputed on this site, and i'm not saying that it does happen, but that is a 'potential' physically toxic side effect of disassociatives which would definitely land them on my 'hard' list if proven.
sorry to ramble, but yeah, i think its up to your own distinction...kind of a gray area. if i was gonna do something that i consider a 'hard drug' i would go for a disassociative over mdma or over pharms or alcohol for damn sure, but thats just me.