I have ~180k miles on my 2012 6.7 Super Duty and the only problems I've had were the EGT sensors, Nox sensor and recently a AC condenser coil. As for the engine and transmission I would say that can go an easy 250k miles.
There's a couple of guys on this thread who like to promote internet amplification and perform the same routine every time so take their claims with a grain of salt. I also have ~20 percent idle hours (>1k hours) and no EGR or soot clogging issues.
I wouldn't hesitate to buy another older 6.7 Super Duty but like any used truck you have to give it your due diligence to give the truck a good look over. I'm quite content with my truck and still plan of keeping for the long haul.