Part of the reason why the blanket advice is almost always "don't do this" is precisely because it's so hard to quantify or predict the extent of the synergy, imo. As
@BridgeInspector pointed out, it's not really 2+2=4, it's 2+2= 4, 5, 6, 7, 8, whatever.
I've also mixed opes and benzos a fair amount of times (and carisoprodol, too; the 'holy trinity' as it's known)... the only time I've had to be revived by paramedics was when I mixed vaporized brorphine and deschloroetizolam.
There's another factor that doesn't always get sufficient attention; benzos impede rational decision making, just like alcohol, and this can lead to reckless redosing that leads to overdose. That's what happened to me; the benzo anxiolysis led to me vaping WAY more brorphine than I otherwise would have, and removed the anxiety I would have normally felt about how depressed my respiration was. Because of this, I blew-off taking naloxone; "I'll be fine, no worries!" *ODs*.
Imo, good harm-reduction practice for mixing opes and GABAergics is to avoid redosing either, and, as has been stated, reducing your doses of both. The synergy can be unpredictable, and can have a dose-response curve that differs from what you're used to using either substance alone. The alterations to the DRC are, I suspect, a contributing factor to benzo/ope ODs.
Lots of people find they mix well, but I've also found it really increases the addiction potential; that synergizes too.
In summation, it's my opinion that you gotta watch out for: respiratory depression, increased potential for psychological addiction, an altered dose-response curve, a narrower therapeutic index (the diff between an effective dose and a lethal dose), and impaired decision-making that leads to bad judgment calls. It's not
just the increased respiratory depression.
Combine all those factors, and you get the reason why most sources advise against it; it requires a delicate balance, and should definitely be worked up to vs diving in headfirst. Lots of people aren't familiar enough with drugs to pull off that balancing act, and fatal opioid overdoses from non-IV RoAs often involve benzos because of people not knowing how to safely manage the combo.
Tbh I don't really think of it as scaremongering; it's incorrect to say it is always a life-threatening danger, but it would also be incorrect to say that it's nothing to worry about. Lots of non-BL users know very little about the drugs they take or the way that those drugs work, and if we think back to when we first started using, we may not have intuitively known that mixing two drugs from different classes could be more readily fatal than taking them separately.
Shit, the number of friends I've had to warn about not mixing alcohol and GHB, for example, is sort of depressing. And the number of people who intentionally or unintentionally killed themselves mixing alcohol and barbs lends credence to the idea that caution and going low 'n slow is a prerequisite to mixing potent CNS depressants.
But yeah obvi it's not the "IT WILL INVARIABLY KILL YOU" shrick some sources claim, given that some people are prescribed both opioids and benzos concurrently. But given how often non-IV ope ODs involve benzos, it defo demands caution
