Why is the sinthetic drug spice so bad? and what are the long term effects?

Many things: for one thing, what is called 'Spice' or 'K2' or anything else is not always the same product. These are illegal factories without quality control where there is nothing indicating the purity and concentration of the product. So, one batch may have five or ten times more of the synthetic cannabinoids. It is also unevenly spread in the product. It can cause seizures and hypertension - pot doesn't.