There are many silly works within the category of music referred to as folk metal. In many cases, the quality of the music and the connection to folk music can both be questioned.
Some culture's folk music would probably not mix well with metal, like some styles of metal would probably not work well with folk music. Heavy metal and black metal seem like the best candidates to me. I think that an attempt to musically speak about the things that metal and folk music can have in common would result in the metallic parts resembling either heavy metal or black metal.
So my questions for you are:
Are there any folk metal bands that you think are good? Let me know why you like a band that you like. And if no band appeals to you, dissect a shitty one and explain why it's bad.
Is folk metal as a style capable of producing great music, or is it a flawed genre?