The illicit synthetics are very dangerous. Completely unregulated and could have been made by any amateur chemist. The main active compounds used for many of them were created in a lab by John W. Huffman, an accomplished chemist who was doing research on Cannabinoids.
Somewhere along the way, people realized they were full agonists at the CB1 and CB2 receptors (THC only a partial agonist at CB1, the receptor which is responsible for psychoactive effects). In addition to being full agonists, they are much more potent as well. This combination can lead to effects that are very different than those seen with naturally occurring Cannabinoids.