inexperianced mechanic is what i would say, its actually opposite petrol engines wear out faster, diesel engines have very strong engine blocks too they can take more milage and abuse, bmw and merc diesels are 3 times over engineered compared to toyota honda, and have even long life easily clocking over 300,000 MILES without overhauling or anything, however these diesel engines in pakistan have very little life and the only reason is the wrong servicing techniques of these mechanics, the traditional approach of mechanics doesnt work with these engines, there are numerour thread on this forum you can check out to get a idea,
manual transmission traditionally used to have better fuel economy however the modern automatics like 7 sod 8 spd or cvt transmissions get much better fuel economy than a manual, the technology has progressed.