They are pretty good for the most part. BUT… I have a couple on the North Vietnamese Air Force, obviously written from their perspective by a Hungarian who appears to take that view as gospel. I am curious to read the ones regarding the Arab Air Forces just to get that viewpoint. But some must be taken with a grain of salt. However most are quite well researched and written with appropriate historical accuracy.
Where I’ve found them really useful is for information about uniforms and personal equipment. Sometimes that stuff is really hard to track down, compared to , say, aircraft.
I think they have maybe four books on the Boer War alone, for instance.
Back twenty five years ago- no internet, Osprey by mail order only, and the “in action” titles by Squadron were about it.