Usually in high power rated modules, FET’s and transistors, smoothing/start-up capacitors are common faulty parts; also due to the power and heat generated, tracking (internal & external) can be a problem, in these cases it’s common to see carbon build up which would require a complete refurb of the area affected.
Other common issues can occur around ICs that are not protected by adequate circuitry and more modern board can suffer from poor manufacture including the accurate placement of ball grid arrays. Mechanically, connectors, and backplane interconnections, as well as plastic locking arms can be stressed, bent, or broken.
Reverse engineering and design improvements to increase longevity is often undertaken and tested to standard field conditions (moisture/high/low temp conditions). This includes changing components to better and high rated spec or modify circuitry and adding additional protection on the unit. These are agreed with our customers on a case by case and part number specific strategy.