I think the problem with this discussion is that pot, like cows milk, is totally different than it was 50+ years ago. Is milk healthy? Not anymore. Not after it was pasteurized and its cows were pumped full of estrogen and given corn, instead of grazing on pasture.
Is pot healthy? I think a lot of it has synthetic additives and most modern strains have "ramped up" the drug effect, with much danger. I've heard many baby boomer hippies say "this isn't the weed we grew up on in the 70s". So did it USED to be healthy/mild? probably, in moderation. But now? You'd have to know what you're smoking. Most of it is laced with God-knows-what. People who smoke weed and have hallucinations...that's just not normal nor explainable.
Sane observations and legitimate questions.
There are dozens of phytocannabinoids and hundreds of terpenes, flavinoids, and other substances in the plant product that contribute to the medical (and psychotropic) effect. For convenience, people often assess potency by the THC content. It's very rough and quite imperfect, but it's done. If you look at the studies, the valued strains of the 60's probably peaked out at about 5-6% THC and "street weed" was typically 2-3% THC. Just checking online, our state dispensaries have leaf hybrids with ~20% THC. Yes, much more potent, but most (certainly not all) patients report ingesting much smaller quantities (a puff instead of a whole "joint").
Adulteration is primarily a problem with the black-market product (primarily pesticides). The advocacy community is
intensely vigilant (as are the control-freaks of .gov/.zog) and so both legal and illegal products are routinely submitted for testing of contaminants and pathogens (especially Aspergillus).
Especially since some brands of the hydroponic fertilizers were discovered to have heavy metal contamination, "outdoor organic" is prized. _IF_ someone qualified (favorable risk/benefit) asked me, I would recommend pathogen-tested, outdoor organic with a relative balance of THC and CBD, say 7%/7% roughly.
To my thinking this is the rub—Almost everyone focuses on THC and CBD, but I think it is clear that all the chemicals of the natural product work in concert, so I think even the flavonoids and terpenes are important. In toto, so much depends upon genetics ("strain"), growing conditions, timing of harvest, manner and duration of drying/curing, and storage, it is difficult to expect uniformity and repeatability.
Do we imagine otherwise for synthetic pharmaceuticals? Last year (2020) Zantac™/ranitidine was recalled because they discovered carcinogens—a problem that had gone on for 20 years.
Barring any unique contraindications, I'd estimate that the risk is less than the risk of driving to the grocery store.