I think drug users like ourselves, and even drug users with a particular affinity for a given drug (a stoner or a weekend MDMA user) are likely to be automatically resistant to any data that suggests that our drug of choice is more harmful than another drug that we perceive to be particularly harmful.
People that smoke a lot of cannabis are sometimes good examples of this (the 'it's herbal, maan' syndrome) and I have definitely been guilty of downplaying the harms of MDMA and Ketamine abuse in the past, until I looked back at my behaviour and presumptions, and realised the many ways in which my beliefs correlated with 'confirmation of existing belief.'
We're given so much propaganda that the real harms of drugs are shrouded in a cloud of bullshit, and this makes us often suspicious of anti-drug information even when it is quantifiable
I've stayed away from the stereotypically 'hard' drugs (heroin/meth/crack) because of my obvious compulsive nature that always lands me in trouble, but personally I have found Ketamine/dissociatives to be the very intensely habit-forming and harmful to physical and psychological health, and also benzos/etizolam.