Most of the time they are :) The only real issue with books like that is that books like that usually have the author scurrying around for references, and once they have one reference, they move on. In such a dictionary, it's almost certainly too much to ask 'em to check out their sources thouroughly, though.

Of course, then you get things like the Miranda Green's Dictionary of Celtic Myth and Legend which has some really, really good stuff, followed by some really outlandish claims. It's most amusing when they cite sources and then come to completely different conclusions :)