The problem is it's incredibly potent. For an adult, you might start at 25 ug. Not a big deal when you're drawing up a 100 ug/mL solution, but it's a big problem when you're trying to eyeball a powder of unknown purity. Plus fentanyl is showing up as an adulterant in street drugs like cocaine, where no one is expecting it.

I was reading that in Canada there were seeing a number of overdoses where the blood levels of fentanyl were >200ng/L. That's an absolutely massive overdose as typical therapeutic concentrations are 1-3 ng/L. People end up needing a naloxone infusion for a day or more until their body metabolizes all of the drug.