- The rankings are creating by combining a lot of different factors that don't have anything to do with each other, e.g. by combining harm to the user with harm to society. This means that drugs like coffee end up being more dangerous than drugs like heroin, simply because more people use coffee than heroin so the total social costs are greater.
- The harms for drugs are measured as they are typically used, rather than correcting for things like differences in demographic and route of administration. This leads to drugs like heroin looking more dangerous than they are, because people who have drug abuse problems tend to gravitate toward drugs like heroin. (Whereas people who use, say, Khat tend not to be the worst of the worst as drug abusers go.)
- The harms of the drugs caused by prohibition are not accounted for. (E.g. they are counting people using dirty needles and impure/unknown/fake drugs as being a harm that stems from heroin, but they aren't counting using dirty needles and fake Starbucks as being a harm that stems from drinking coffee.)
- They're not accounting for the benefits of drug use, only the harms.
I could keep going, but I think I've sufficiently proven my point and then some.