6.7L is the Ford diesel motor they came out with to shut everyone up b/c the two before that had issues. They drove the 6.7L for one million road miles at the proving grounds before launching production. The were making sure it was right b/c they couldn't afford another problem. They had to save face. Saying that though there are three versions of the 6.7. 2011-2014, 2015-2019, & 2020-2021. Mine is a 2015. The only problem I have had with mine was the fuel / def heater. I had to have it replaced but then it ended up being a recall and they reimbursed me for it. Some people are ate up with DEF being dumb and wanting to delete it off the truck. I can see how it would bug someone that grew up not doing that or something but its really not that big of a deal and I don't buy into the whole get a chip and delete it thing, I'd rather get 300,000 miles and not have a warped head like I've seen with guys running them too hot.
The 6.7L is notorious for pulling things just fine like they aren't back there. I have had no trouble with mine and have had it for four or five years. Its got 130,000 miles on it or so. I really like it.
Point being, check out what was changed from 2014 to 2015... thats what the weak spot was with them which actually answers your question.
I looked it up b/c I was curious looks like from 14 to 15 we got about another 40 horse and the addition of the jake break which is actually pretty handy pulling a camper.
The 14's were setup to shine at low end pulling and the horse power move to 15 made it more sporty on the higher end it looks like.
https://www.powerstroke.org/threads/difference-between-2014-6-7-and-2015.923714/