The amount needed to for a fatal overdose can vary wildly between people. Current opioid users can have a large tolerance. Also, suppliers may not supply a pure substance. It can take a bit of time (not sure exactly), and if found the effect can be reversed. Also highly illegal. It is said that adding grapefruit juice makes it even more effective. Carfentanil is many times more potent again, to the extent that there are concerns with ensuring nobody else inhales even a tiny amount, including first responders.
Aside from all that, it's one of the best methods - easy, zero discomfort, affordable and euphoric. Would definitely be my choice if supply weren't an issue.