IMO its just less likely with smoking, since one is continually adjusting and taking more, at a lower rate than with injecting, or railing a bolus dose. And once its in there you can't take it out, with a shot, its all in there in one sudden go, rather than more gradually topping up with a pipe full and tooting from that. Its not impossible to die. Its just made less easy.
And if fentanyls come into the equation in any other sense than 'aren't present' then that goes straight down the sewer with regards to safety. I'm a chronic pain patient, as well as familiar with some rather pure (uncut, and I could verify as much since it never left my sight after acylation) dipropionylmorphine, H, dibenzoylmorphine and the like, in strength, and it was present, could taste the H, but some came up that smoking so much as a half rice grain sized portion, 10mg or so of street powder gear was enough, for one or two tokes of the pipe to knock me out cold for several hours.)
Pretty sure it killed my connection, as well as dealers he went to, since after it surfaced, about a month and a bit later, EVERYBODY I knew that had touched it wasn't around anymore, never heard from again. And I'm bloody certain as I can be it was this stuff thats responsible, and that I think it did them all in. ALL of them. Every last person in any way buying, using, delivery lads and dealers, homeless users with high tolerances and long habits. Not heard, after that bit more than a month, of a single word from a single one. Or heard from anyone that has heard of them alive either. No deaths confirmed, nobody confirmed alive. Just gone, like a silent but lethal storm had sent a thunderbolt arcing from person to person to person, felling them all.
Of those I know of, that touched this stuff I am the only one I know to be alive. And that I put down both to knowing about fentanyl and its kin, and being by interest, I'd say a quite well read, and well-educated and aware toxicologist and pharmacologist, I recognized almost immediately from the first hit or so that there was something else than H going on there, warned both the users, dealers and had them pass the message on, also warned professionals, not who was slinging, but the fact that this stuff existed and how potent it was, duration of effect, appearance etc. so that warnings could be circulated in clinics distributing free works to the homeless etc. could put up warning signs, give verbal warnings through some covert channels, word was put around at least. But still, the epicenter of the appearance, from my perception, it was ground zero, just as effective as had a nuke gone off in the city center, with me on the other side's far outskirts to the city limits more or less it had hit. And with a bomb shelter. I'd gotten about ?140 worth of the stuff, not counting some free bags included for that in one order for the size of it, and that, had it been 100% pure dipropionylmorphine the doses that would KO me in one vaped hit, wouldn't have had any physiological effect at all, only taste perception, and the bags would have lasted me a week or so; that stuff lasted months, and one bag many days.