What makes fentanyl and fentanyl analogs so dangerous is their potency. A dose is measured in micrograms and even as little as 1mg (or less depending on the analog) could kill you if you dont have a big opioid tolerance.
I don't know which fent analog is the most popular at the moment, but they're a few cheap ones like carfentanyl which are extremely potent. Fent analogs are very easy to synthesize (unlike morphinians) so they're easily accessible, plus they're really easy to smuggle too since you need so little to feel the effects (compared to other opioids).
So yeah many people overdose because they think they're doing heroin and some dealers don't know how to cut it properly so you get "hot spots" of fent (meaning it isn't properly mixed with the cut, which is a very difficult thing to do).
Otherwise fent and its analogs are generally very very selective towards the opioid receptors and specially towards the mu-opioid receptor subtype (agonism of this receptor can cause respiratory depression=death). So they're way stronger and potentially more dangerous than most popular opioids.
However if dosed properly they're not more dangerous than any other opioid, plus they also have fewer side effects since they're so selective (less histamine release for example).
Don't have any personal experience but from what I know it seems like fent is less enjoyable than traditional opioids, clearly mu agonism isn't everything regarding recreational effects.
