Osprey books are really good as an entry point into military history I've found out. And you can't deny most plates look very nice.

But yes, the main criticism is the sources used for plates and sometimes shoddy research (according to some. In my opinion Osprey seems to do a good enough job with my most books I've read). Sometimes the sources are used very well, but other times they are taken from various sources and time periods. An as russia almighty says, equipment worn by Kings or high nobility are depicted on common soldiers in some illustrations.