Dude, yes it is.
Or at least, in many cases it is. This is my opinion after about 30 years of being around pot smokers and being one myself.
Also, it's a matter of degree. I know of many folks that smoke daily and still lead productive lives. But I believe those to be in the minority. Many more I know are in their 40's and still live with their parents or can't hold a job etc.
Then there are ones like me that wake up one day and realize that they've spent the first half of their adult lives in a haze of pot smoke and are nowhere near where they want to be in life because of it.
But I'm feeling much better now...
Dude, you've just disproved your own point. If it IS the drug, we'd expect to see little variance in it's ultimate effects.
Losers tend to overindulge in self-destructive behavior, of whatever type.