These engines are hyper-sensitive to proper build and set-up. When done right (and left in a stock or near-stock condition in terms of power), they can be very reliable. Mods that are done for longevity are acceptable; mods that significantly turn up the power are just a problem in the making. Improved oil coolers and EGR coolers, as well as proper FICM and related improvements like voltage supply and control, are a good thing. Tuners and fancy bling which mask underlying problems are just denying the inevitable. As a general statement, my opinion is that no more than 350 hp and 600 ft-lb makes for a reliable 6.0L engine, as long as the proper mods are done for longevity. And in fact, tuning the engine down from Ford's numbers (325hp; 570 ft-lb) to lower values not unlike the offerings from Navistar, make it that much more reliable. We all know there are Achilles heel issues with this engine design; aforementioned items. Others, such as the HPOP and related galley o-ring ... well, some of them just need long-term care because of the choices made in engine design. For example, the HP galley oil O-rings will need replaced, and the updated D style can help a bit, but eventually, all o-rings have to be replaced due to heat-cycle aging. It's no different than replacing leaky gaskets. The major issue even with a reliable 6.0L engine is that it's expensive to work on due to design choices; it's not nearly as "simple" as the old venerable 7.3L PSD. Things fail on the old 7.3L, but they are pretty much all external, easy to get to, and fairly inexpensive to fix (leaking turbo pedestal o-rings, oil cooler o-rings, fuel bowl heater, cam sensor, etc). With the 6.0L, even after you "fix" the oil cooler (buried and expensive), and the EGR cooler (expensive), you're still looking at HPOP updates, oil galley seals, expensive injectors, etc in the 6.0L. It can be a very reliable and strong work engine AFTER you spend a lot of time/money on it.
There are many major-market re-manufacturing places that, IMO, don't do a good job of the re-man work. Most of them have become just fast-paced, profit driven clearing houses for parts/engine turn around. Rather, it's far better to seek out a qualified reputable shop that will re-man it locally, and has the reputation of standing behind their work, and is recognized as a long-term local community business.