Given that the C file streams don't have a mechanism to replace the back end at all, I don't see why you think that not being able to predictably interact with the back end in the C++ streams would be a disadvantage for C++ streams. It seems extremely illogical to state that product A is inferior to product B because product A won't let you do something that you can't do with product B.
Right, but I only ever use C streams as a back end. Are you just saying that this is a task that cannot be attempted?
To clarify the clarification, I use my own stream model, which is a third and completely separate stream model in regards to the two standardized ones mentioned here. I do have a file stream type, that uses C standard I/O to manipulate files using my stream interface.