I have been doing 40 or more flights a year for the last 10 years, and amassed a lot of miles prior to that. All on established airlines, and I've never seen a case of drunk, awkward passengers, assaults or other troubles.
I must have been lucky. I do make a point of thanking CC and flight crew on leaving, even when (as on most US based airlines, the service is cr*p). After all, they are just as much qualified professionals in their field as I am in mine. In that time, I've had one go around (big deal - interesting, but that's all), and two pax taken ill and really well looked after by the CC.
So are there specific routes where trouble occurs? Or have I been lucky?