I had 2 2014 Promasters, now I have 1.
The first one, which I still have, is a diesel I bought brand new. It went into the shop with 65K on the clock because it was leaking oil from the rear main. It ended up that it had blow-by.
It was my main service van. They kept it for 3 months before finally replacing the engine and getting it back to me.
During that 3 months, I was running my other 2014 PM which was a gasser. It developed the Pentastar "death tick." I nursed it along as it got worse and worse, hoping the dealer would get my diesel back to me, and I could put the gasser in for the warranty repair. As days turned into weeks and weeks turned into the months, it got worse and worse.
Finally, it got so bad that it was about to give it up. I pulled it into a Nissan dealer and traded it for a New NV2500. I had to drive it back home and pull the shelves out of it, and I just knew it was going to implode the next morning when I drove it back to the lot.
I still have the diesel. The shop not only did not put the correct Eurospec oil in the brand new engine, they didn't even put full synthetic. They put conventional/synthetic blend in it. It took me 4 oil changes to get all of it out of there. It kept giving me an oil viscosity trouble code, and would go into limp mode.
To me the most important issue BY FAR is reliability. I think it is ridiculous that you cannot rely on these vans to give you a bare minimum of 100K without needing major repairs.
We have friends that have had problems with the Transits too. Nissan is the only manufacturer that expresses confidence in their product, as manifested by the 100K bumper to bumper warranty. FCA dropped the warranty on the Promaster from 100K powertrain to 60K after the first year of release. They saw that handwriting on the wall before they started selling the 2015 models. Incidentally, the "powertrain" is very limited. It covers only parts that oil touches.