The Ford diesels pulled extremely well, but lacked reliability. I towed my last loads with my 08 550 in January 2020. Had a couple loads that were close to 38,000lbs. No gasser would have made the journey.
She would pull, but she would throw check engine lights a couple times per year and would leak, drip, etc. it was a high performance, very cool design engine with a strong bottom end, but Ford went cheap on rocker arms and other areas and in the end, it cost them a lot of long time customers, including me.
To own a 6.4, You have to know the engine well. You have to know how to care for it and maintain it. Otherwise it will be a less than satisfactory ownership experience
Heres a 38,000lb GCWR load