Apart from some questionable organization choices across the varying books ("Monsters" and "Giants" have noticeable overlap, and some monsters could feasibly be dragons) and a tendancy to favor Greek and British lore, the books are an interesting review of fantastic creatures from stories around the world.
My primary concerns with the series as a whole:
1. All of the stories are refered to as myths—whether or not they actually belong to that genre. A myth is a specific type of story. It may share cultural space with folklore, fairy tales, and legends, but each type of tale has distinct goals and hallmarks. Using the term "myth" to refer to all of those types of stories dilutes the power of the word and also makes it more difficult for younger readers to grasp the more nuanced connotations later in their educations.
2. The writing conflates fantasy and reality in a strange way, with sentences like, "All dragons have scaly skin."
3. The only glaring fact-based problem that leapt out at me was the identification of Beowulf as a myth that comes from Denmark—no mention of it being an Anglo-Saxon epic poem written by an Anglo-Saxon author in Anglo-Saxon England. The oversight makes me wonder what other information is misleading!