I've had my 6.4L diesel for 3 years and did a TON of research on the engine before / during owning it. Maybe I'm just lucky but I never had a single issue with the engine - that said the previous owner had immaculate maintenance records and I've been very diligent about the maintenance schedule as well.
On the good side despite the bad press on the 6.4 I get decent mileage for an RV, the engine is super powerful, and I love the Tiger form factor as it's a dream to drive long distances or off road and feels bomber in situations where truck campers feel super tippy.
Most common issues on the 6.4L diesel:
-Most of the complaints online seem to come from the DPF system causing a number of downstream issues. Mine works flawlessly so I never messed with it, however lots of folks have seen good success with a DPF delete (depending on your use case / where you are located if it's legal.)
-The plastic on the end of the radiator is a bad design and over the long term can start to leak.
Here's a good and very comprehensive article on the all of the 6.4L potential issues and solutions + aftermarket parts - again the majority have to do with the dpf system:
https://dieselresource.com/diesel-resources/6-4l-powerstroke-problems/
I'm definitely biased as I love this truck and am currently selling it, but in 3 years and some demanding use it's not had a single engine issue.
Like anything else, I think doing your research and being educated before buying (for ex I would not buy a 6.4 that didn't follow the maintenance schedule, and I'd recommend oil changes every 5k miles vs 10, but that's easy) and knowing what the potential problems are is most important on the 6.4. If I wanted to bulletproof mine I'd look into a DPF / EGR delete, but because the mechanics I've had service it say it's in great shape I haven't bothered. Although it's allegedly an animal with the dpf delete with 500+ hp
I have it listed on the site here because I'm getting married and weddings are stupid expensive, but otherwise would keep it forever:
https://forum.expeditionportal.com/...les-86-500-obo-denver-co.239857/#post-3115028