Fentanyl is not a safe drug. Period. It is a powerful and dangerous drug with dangerous and potentially lethal side effects and classified as such. It's also and incredibly useful drug, potentially of great benefit to people in severe pain - benefit that must be balanced against the danger and risk.
In controlled settings (e.g. in a hospital bed with physiological monitoring) many dangerous drugs can be used in a safe way, Fentanyl included. But that doesn't make Fentanyl a safe drug, just a drug that can be used safely.
In the same way TNT and rockets can both be used safely - neither are sensibly intrinsically safe.
> If someone overdosed and you breath for them (CPR or ventilator), you can just wait for the effects to wear off and they’ll be none the worse for wear.
I think this is too complacent. Aspiration is a real risk, they could choke, pneumonia can be life threatening. If somebody has overdosed, get professional medical care - management of the compromised airway is no DIY project.