^^ No, it isn't. You bolting a 6.2 liter LS3 into a Fuso, a Mustang or a Honda Civic and Isuzu bolting that engine into an NPR are quite similar in nature. With the after market support that is available for LS-family engines, GM/Duramax and Dodge/Cummins diesel engines, I would contend that third party and after market builders often create higher quality, more reliable and less cumbersome final products than do the OEMs.
The engine, transmission, transfer case, axles, dash, headlights and so on, need certain 'things' to run, either mechanically, electrically, or both. Provide this data using a home-made bracket, harness or adapter or one purchased from a dealer and the engine doesn't "know" any different, nor does it 'care' that it started life in a Silverado 2500 and is now under the cab of a Fuso FG. Sure, people hack things together for this reason or that, but a properly done swap should be perfectly drivable and reliable. It only takes a trip to the service department of a new car dealer, save Lexus, and a chat with a mechanic there to discover how complicated, trashy and unreliable modern vehicles are.
The Duramax swap into the FG is brilliant and if Isuzu had any sense, as Duramax is a joint-venture between themselves and GM, they would have offered the 6.6L diesel V8, an Allison 6 speed transmission and the 263 XHD transfer case (from the SIlverado 3500) after having resolved the pump rub issue, in a USDM Isuzu NPS. THAT would be quite cool.
