Its not so much that the 6.0 is a bad engine, I've had 2, but more about just how ridiculously expensive it is to repair the modern diesel when something goes bad. When the high pressure fuel pump on my 6.4 went south, sitting it my driveway, it was an almost $8,000 dollar fix. Dang near double that if done via the Ford garage. When the HPOP blew on my first 6.0, $4k at Ford because it was out of state and I didnt know where else to get it done, and that wasnt the end of that story because when the HPOP blew it fragged some injectors in the process. Of course Ford didn't know/care about that so I had to have all those replaced by someone else. Current 6.0 runs like a raped ape, but the first thing I did on purchase was take it to my favorite PStroke shop and get all the usual bullet-proofy stuff done. About $6-$7k in preventative maintenance.
I can buy a complete set of injectors for my 7.3 for the cost of 1 for the 6.4 I had. I can buy a complete 7.3 powerstroke long block from Ford for about the same money it cost me to fix that fuel pump in the 6.4.
That's the problem with the 6.0, 6.4, the 6.7, and just about any other modern diesel.