The UK is seeing a plague of synthetic drugs that share some properties with cannabis. The problem is that while cannabis is a selective CB2 agonist (CB2 receptors are mostly in the brain), these synthetics are not selective thus their effects are much more pronounced and unpredictable. We are only beginning to learn the risks.
I would also like to point out that I have seen dozens of GC-MS & NMR results from these synthetics. As well as expected impurities, dimers, trimers & polymers are apparent in almost every sample. Now, these compounds have a much high MW and a much higher melting-point & boiling-point. That means that when you smoke them, instead of being vapourized, they are pyrolyzed producing a number of chemicals that are known carcinogens. Time will tell us just how this story ends but it is expected that their will be a spike in cancers associated with these synthetics. Just because they are technically legal does not mean that they are safe.
It isn't for me to tell you how to live your life, I just think it's reasonable for people to know the risks. I will finish by noting that my concerns were ignored. As long as the product looked like a clean white powder, the trader was happy to sell it on. I presume that almost everyone trading in these compounds either doesn't know or doesn't care.