Synthetic is not invariably a criticism around here. When used in a negative sense, it typically implies one of the following:
(1) inferior replica of a natural component
(2) unwelcome suggestion of man-made materials such as detergent, plastic, medicine
While individual views vary and some of us have commitments to all-natural perfumery, I expect the majority of us (myself included) would agree that modern perfumery has been greatly enriched by synthetic aromachemicals.