Back to one of the earlier responses, the .38-44 cartridge (essentially a more heavily loaded .38 Special cartridge) appeared in the early 1930s. It is usually considered to have produced a peak chamber pressure of at least 25Kpsi, far higher than the peak chamber pressure of the modern +P .38 Special cartridge. Yet, there were never any warnings given by either the firearm or ammunition manufacturers that it was dangerous or inadvisable to use .38-44 cartridges in ANY .38 Special revolver available at that time, even the earliest .38 Special revolvers. The only warning given was that .38-44's recoil was somewhat greater than standard ammunition.

I am no fan of +P cartridges and do not advise their use but that is not because that it is unsafe to do so, as it is not. I just consider them as being sales gimmicks.