Yeah, of course it's a great advance in harm reduction to have Narcan available for opioid overdoses, and a nasal spray would be even better!
The thing is just that these substances (naltrexone as well as naloxone) are not just potent opioid (mu, delta, kappa) blockers / antagonists but in fact
reverse agonists. This would explain why, once adjusted to them, they can alleviate cravings for you. But it also means they cause a precipitated withdrawal that is far worse than plain antagonism, or discontinuation of the opioid, would do.
So we should have a neutral, plain (but very potent) antagonist. Or even a equally potent partial agonist with such low intrinsic activity that it would block all the opioid receptors, kick the overdose off them, and very slightly activate them by itself to a safe degree that doesn't lead to respiratory depression but alleviates the worst part of withdrawal symptoms.. somewhat like buprenorphine 2.0.
Also some people seem to have a genetic variant (?) leading to changes in endorphin transmission or somewhat. This has been associated with various mental disorders, including addiction, but mostly the depressive / overly sensitive etc. direction. For those, taking an opioid inverse agonist even when they never did any true opioid before, can be a terrifying experience.. like it was for me.. this is just a theory, but in my eyes a plausible one as the endorphin part is backed by study evidence.