Some synthetic cannabinoïds are pharmacologically different from THC. I've once taken 3 grams of realy potent hash, and the major side-effect was extreme drowsyness. But it wasn't unpleasant at all. No sickness. If you realy want to get serious OD symptoms from THC, i don't know how much you should take, but it must be realy bizare amounts.
Some synthetic cannabinoïds aren't just much more potent, but also much more dangerous. THC is a partial cannabinoïd agonist. Some synth's are full agonists. That's realy a wholy different thing in terms of risk.
I believe that some synthetic's are realy dangerous.
Cannabinoïd receptors are probably the most dominant receptors in the human body. In numbers that is. They regulate GABA, serotonin, dopamine, etc and you'll find them in virtually every human cell. Full agonists can realy mess up your Chemistry if you take too much of them. It's not the same as partial agonism. People have been hospitalised because of taking too much of them before. They're also much more addictive.
I think synthetic cannabinoïds are realy a serious concern when it comes to public health. There are places where this stuff is being used as if it's just cannabis, mostly because of the dodgy legal status. But given how addictive the stuff can be, AND how realy damaging some of it can be as well when used excessively (addiction and excessive use is something that tends to coincide), i think it's realy unwise to treat the stuff as if it's equal to cannabis.