They typically very reliable. The late model ones (17+) seem to have greater fuel dilution issues and more frequent regens compared to the previous generation. We have a 2013 6.7 that is 100% stock with all emissions and nothing has ever been touched. No sensors replaced, original EGR cooler, HPFP, injectors, everything. It has 19500 hours on it. They are serviced every 4-450 hours. I'm sure it's a matter of time until something breaks, though.
Pay attention for radiator end tank leaks mishimoto radiators are nice but expensive replacements., use a high quality oil, I also suggest optilube fuel additives. They will help keep the fuel system lubricated. I also highly recommend OEM fuel filters only.