Sometimes brass the limiting factor
There are some instances where the brass cartridge case does really limit the max pressure. The old black powder balloon design .45LC is an example. Don't load those with smokeless powder to the max for .45 LC in your manual. Of course the gun is an issue to. An old 1873 Colt made for black powder isn't going to like it eiither.
A .30-40 Krag case (designed for smokeless powder from the start) can't be loaded up to .308 or .30-06 pressures). Both the gun and the brass are limiting factors. The old .30-30 has been made for smokeless powder from the start but the case design is the real limiting factor. Your old Winchester 94 is limited in pressure. But there are a few bolt guns and at least one lever gun (the Savage 99) that were made in .30-30, that can take modern high pressures, but that doesn't mean that you should pack your .30-30 brass with enough powder to approach .308 or 300 Savage performance. The cartridge design just won't take the pressure.
With the ageless .45-70 you can load modern commercial brass to a very wide range of pressures (low for Sringfield Trapdoors, Medium for Marlin 1895, and High or Ruger and TC single shots) and the the same brass is used for all classes of gun. I do have to be careful because I have several .45-70 rifles and I have to take great care not to use my Marlin loads in my 1873 Springfield Trapdoor.
The 9mm was made for pretty high pressure right from the start and so uploading it to +P pressures is not a big deal assuming that your gun is in good condition and rated for it. The notorious NATO 9mm is too hot for many handguns but the brass isn't the issue. The gun is.
A lot of guys, myself included, load .38 special cases to +P pressures and it's certainly no big deal for the brass. Of course you would want a gun rated for +P in order to do that. You could probably load .38 special to .357 mag pressures (using starting loads for that caliber) and fire it in a .357 revolver (although neither I nor anyone else is going to tell you that it's OK to do so) with out over stressing the brass.
The point is that with .38 special and 9mm there isn't anything special about brass that is stamped "+P". The brass can be a limiting factor (like the .30-30 and the .30-40 Krag) but often it is not.
Sorry, I think someone asked me what time it is and I just told them how to build watch.