You can not prove that weed has no stand alone impact on mental issues, while it does seem to have a stronger accepted link with people who were apparently ''on that path'' I tend to think that is dismissive. You could argue certain soldiers are more prone to ptsd, I'd tend to agree. But you could also argue that the ptsd was very unlikely to have arose in day to day life, war or the likes was above the threshold trigger for said people, so they lost it. I think the same is true with drugs, one person is fine on E, the next not, other fine on heavy canna use, the next not. Also is the complicated issue of drug mixing, many do it.. and understanding how such drugs may interact with each other to worsen possible side effects is anyone's guess.
The real problem with identifying the links between drugs and mental issues is you can not put the same person back in time and test if they were to have issues while not taking X, Y or Z drug. Yet, we do know that heavy drug use of any sort has a correlation with mental issues, be it bringing them out almost ptsd style or not.
Never at any point did I say the severity or frequency of canna side effects are as prominent as other drugs but it is a poor, dismissive argument to condone the over use of cannabis on that basis. Cannabis does have side effects and as the strains/concentrates get stronger and stronger the frequency and severity of those side effects will rise, and then the frequency will rise some more if fully legalized because more people inevitably begin using it. You have to keep in mind, when we were new smokers even so close back as 10 year, dope and low grade weed was everywhere, this meant your body and mind had time to adapt. The kids and even new adult smokers these days have a lot stronger strains (even if only from being better grown) and ever increasing strength of concentrates. I do believe those concentrates especially, will be the prime suspect for triggering the ptsd style shock into psychosis for novice or liberal smokers who get too heavy too quick. Butt hat's ofc worse case side effect, the more frequent side effects are once again over looked as meaningless. Becoming anti social from too much smoking is not a good thing, it's very damaging to family, friend and work ties, to the point continuation becomes life changing.
For the record though, if I was forced to choose the drug my children would one day become addicted to, it would most definitely be cannabis. But that is cannabis in it's most natural state. Weighing up the pros and cons of it and other drugs, it is the lesser of most evils.. but that doesn't make it right. Heavy cannabis use for no good reason is not a good idea.