With ReadFile/WriteFile you have deterministic error handling semantics. When you use memory mapped files, errors are returned by throwing an exception.
In addition, if the memory mapped file has to hit the disk (or even worse, the network) your memory read may take several seconds (or even minutes) to complete. Depending on your application, this can cause unexpected stalls.
If you use ReadFile/WriteFile you can use asynchronous variants of the API to allow you to control this behavior.
You also have more deterministic performance if you use ReadFile, especially if your I/O pattern is predictable - memory mapped I/O is often random while as ReadFile is almost always serial (since ReadFile reads at the current file position and advances the current file position).
