The Ospey books, by and large, are excellent resources and used by many historians as a "quick reference" in disciplines other than their primary. Their are a few misses and less than ideal ones (such as Konstam, with whom many priate historians disagree, as well as some of the highlander ones). Even on the "bad" ones, it is the result of one historian writing it, while others disagree with his/her ideas and hypotheses.

The Pict one is actually one of the best I've seen and it presents the various angles of disagreements and general consensus quite well. If interested, I'd HIGHLY recommend it.