Rebuilt engines, which are sometimes called refurbished engines, have had broken components replaced, so older parts that were not replaced can still fail. People performing the work might also inspect parts of the engine that were not replaced to ensure that they're not suffering from obvious mechanical problems.
Engines are complex, and the sheer volume of critical components makes it difficult to estimate when a part is going to fail. While experts can notice signs of wear and may run tests on the engine, some problems, like metal fatigue, might not be visible. In addition, mechanics are often paid only to service what clearly needs to be replaced. The mechanic might not be paid for opening a part of the engine and inspecting its components if the repair work doesn't require doing so.
Another option to consider is remanufactured engines. These engines have been reassembled primarily from unused parts, and they also contain all of the upgraded components the manufacturer has switched to over the years. As a result, experts often state that remanufactured engines are generally better than new engines, which may sit on a shelf for years. While remanufactured engines typically cost more than rebuilt engines, they often come with fairly robust warranties; this guarantee alone may be enough for some drivers to prefer remanufactured engines over rebuilt engines.