The PLoS ONE article says what they studied:

We counted participants as having any lifetime psychedelic use if they affirmed use of LSD, psilocybin, mescaline, or peyote. We also examined use of each of the substances separately. Mescaline and peyote was combined into one category "mescaline/peyote" because mescaline is the active substance in peyote cactus, but peyote was also examined separately. Information was also available on past year use of LSD, but not past year use of psilocybin or mescaline.

So, they didn't look at cannabis.

My own anecdotal sample-of-one experience is consistent with this: LSD and psilocybin caused no problems longer lasting than brief confusion/panic and a mild hangover the day after, whereas cannabis could put me in a fuddle-headed downer for weeks. (I haven't tried either for many years). Neither came anywhere near having the long term depressant effects of inhalant anaesthetics, either nitrous oxide used recreationally or heavier stuff used for surgery.

As several posters here have said, potency of a drug doesn't determine how smashed you get on it. Most users of LSD these days take much less at a time than was common practice in the 1970s, and the main effect of cannabis being available in higher strengths is that it's easier to conceal a smaller volume - people don't get any more off their heads on it than they ever did. In most areas of pharmacology, a highly potent drug is desirable, because it's less likely to have side effects than a weaker one where a lot of the chemical is doing biochemical stuff irrelevant to the intended purpose.

The PLoS ONE paper:

http://dx.doi.org/10.1371/journal.pone.0063972