One disadvantage of using threaded connections to join pipes and fittings is that it:

The choice indicating that threaded connections weaken the pipe is valid as a disadvantage. When pipes are threaded, material is removed from the pipe to create the threads. This cutting can disrupt the structural integrity of the pipe, potentially making it weaker in the threaded area compared to the solid material of the pipe. Additionally, stresses can concentrate around the threads during operation, increasing the risk of failure, especially in high-pressure situations.

The process of threading can also lead to minor variations in pipe wall thickness, which can further compromise strength. In applications where pressure fluctuations or vibrations might occur, threaded joints can be more susceptible to leaks or failures. Therefore, while threaded connections are straightforward to use and offer convenience, their potential to weaken the pipe is a significant concern in many engineering applications where reliability is crucial.