Portable and window air conditioners are small, self-contained devices, while central air conditioners are larger and more complex, so repairs are generally more involved. Central air conditioners are also more expensive, so even expensive repairs are more likely to be worth the cost over purchasing a replacement.
Almost all air conditioners contain similar parts, and appliance owners who conduct a bit of research can sometimes diagnose their own problems. This is often simpler on portable and windows units since the relevant parts are contained within a small space. Experts can generally diagnose problems quickly, and the small size of portable units means owners can take their devices to a repair expert rather than needing the expert to visit the home, which can result in lower repair bills.
Central units are generally tailored to specific structures, so experts need to spend time learning how the system is put together. Since central air conditioners are expensive to purchase and install, homeowners might stick with an older unit for longer. Replacement parts may become be difficult to locate over time.
However, central air conditioners are generally designed to be easy to service. In many cases, air conditioners are placed in large attics or other rooms, which gives repair experts plenty of space to work.