Of course they won’t save you from external data. The whole point is for your system to have a way to identify rows internally so you can deal with external sys

Of course they won’t save you from external data. The whole point is for your system to have a way to identify rows internally so you can deal with external systems getting wonky without corrupting your own data. All of your concerns are easily solved by a unique index. Using external keys as a forcing function to prevent people from representing data wrong is not great.