I don't think it has much to do with synthetics versus naturals. Many synthetic chemicals are also found in nature. an obvious example being vanillin, which can be manufactured but quite naturally occurs in vanilla extract.
That being said, there is something in Parfumerie Generale's Bois Blond that makes me allergic to it, and it seems worse each time I'm exposed. My sinuses clog up, my nose runs, and I get a rash on my skin. Not nice, yet otherwise it would be an all-time favorite fragrance.