I don't think it you can simply say that Embedded development Board's have advantages and/or disadvantages. For the most part, they are very advantageous tools that a student or an engineer can use to learn about microprocessors. Whether it be an introduction to the topic or for hobbyists, even up to industry level prototyping or experimentation for engineers.
The dev board creates an easy to learn platform that already includes components for interfacing, input/output, a power circuit, and other peripherals depending on how advanced of a product you are using.
Ofcourse they aren't perfect, and will have limitations, but I wouldn't call it a 'disadvantage', it would just be a limitation of that certain product, and you could always look for something that can offer what you're looking for. Again, 'Development Boards' is a very broad term that encompasses a number of products.
I think a better question would be, "What are the advantages/disadvantages of [insert name of product] dev board?"
You might find this useful:
"What development board to use? (Link)":http://hackaday.com/2011/02/01/what-development-board-to-use/