Theres no such thing as a "reliable" torque specification- it doesn't exist.
If you can't trust the factory service manual, you can't trust anybody. I've been torquing fasteners for 40 years per factory service manuals and have never had any issues at all.
Simple, read what I wrote again. ( been one of those OEM's who wrote them specifications for some equipment)
They already know the average person will most likely not have the tools and equipment necessary to do things properly.
They also know theres a point where liability ( both brand and civil) attach ( so legal reviews them too) attaches.
They also know they have to give you something to go by for maintenance etc.
So they do controlled experiments with whatever the standard is applicable and come up with a safe comfortable range and publish it.
Then theres the fine print( expressed or implied) that defines that specification ( certain finishes, clean holes, proper threads, proper flatness etc.) that gives them numerous outs.
I can also show you hundreds of times ( many in court) where OEM 'specifications" were not only flat out wrong but actually caused the damage so an OEM is anything BUT an "infallible authority" on anything. Many of them just print industry standard settings with little to no testing to validate it. Others farm it out and base specifications solely on modeling like FEA or Solid works with no actual lab testing.
To your point, published specifications are generally adequate for all "normal operations" ( whatever that is) and they work. Nobody disputes that.
However context is key- the OP asked about "reliable specifications" in general and that's what I addressed specifically.