Churchill is reliable as a primary source, but not as a historian. Almost anything he wrote is going to be 60-90+ years old, and no historian of that vintage (or very few) can really be considered reliable. Modern perspective is so different and modern scholarship so much more extensive that any recent account is bound to be more reliable. Churchill isn't an exception.

On top of this general caveat, Churchill is a victorian by upbringing, a romantic by temperament, and a politician by occupation. He's the sort of person who has a unique perspective rather than a necessarily reliable one. So even if we could generally rely on authors of his generation, I wouldn't rely on Churchill particularly.

Having said all of the above, Churchill's writings are often referred to as a source by historians of the twentieth century. His works include personal reflections and access to state records that are quite useful to understanding what the governments he participated in thought and why they acted as they did.

In general I wouldn't repeat a statement from Churchill as an established fact, without first confirming it from a more modern source that has already done the work of scrutinizing and critiquing what he wrote. (Also that modern source shouldn't be William Manchester's biography.)