We purchased 3 trucks with gliders. (2002 engines in a 2014 body) The idea was sound but...
PROS:
-remote shops know how to fix the engine
-most repairs do not need a computer
-no EGR, DEF, SCR, DOC maintenance.
-truck will usually run for months with a check engine light on
-the drivers we have prefer them
CONS:
-some engine parts are hard to come by because they are not stocked
-some shops will only use OEM parts - which are harder to come by.
-if a computer is needed, some shops do not have the software that far back.
-the glider engine came with mostly refurbished parts that failed quite quickly (turbo, water pump, belt idler)
The biggest problem is the part availability.
OEMs are required to produce parts for 15 years but nobody enforces it. We have waited days on parts as they come from DC's in remote locations or they source someone to make it.
Once we needed a 9 rib belt after a water pump failed and they wouldn't put an 8 rib belt from NAPA because OEM specs said it required a 9 rib. We had to drive the truck out, call a different vendor to obtain the 8 rib belt from NAPA and then put it on for the driver to continue his trip.
Other than that, we're somewhat ok with it. It wasn't our best decision but it wasn't the worst either.