How's this for irony... Heroin has been around and in use for well over a century; morphine and related compounds (i.e. raw opium) for much much longer. Heroin\morphine have been used extensively and researched quite extensively within the medical\scientific community for the past several decades such that we know how such compounds work and what kind of effects present with short-term\long-term usage. We know that morphine and related compounds are psychologically and physically addictive (stopping abruptly after prolonged use results in a physical withdrawal syndrome). We know morphine is a depressant; it slows breathing and heart rate and can do so to the point of death at high doses and in less tolerant individuals. We know morphine will cause your pupils to constrict, it also causes constipation which can be severe. Morphine in particular may also possess some degree of hepatic and renal toxicity if used long-term, perhaps more-so in those who already have liver\kidney disease. And we also know what morphine\heroin do not typically do... such as cause brain damage, damage to the heart, or any other major organ (this is one reason why it is so frequently used in medicine).
These research chemicals, on the other hand, have not seen such wide-spread use (not even in lab animals!). Some of these chemicals may have been first administered to humans less than a decade ago. Their long term effects, especially with frequent use, are entirely unknown!