There are indeed a few things to consider:
1. Making sure you only skip overwriting files that didn't change at all. For example, the directory to copy does not only contain updates, but also WOU supplemental files. Those must be overwritten as they will be updated frequently. Updating the catalog file wsusscn2.cab is an absolute must or the new updates won't be installed!
2. You will lose out on the WOU feature that cleans superseded and no longer used updates. Simply copying new files to the target won't delete those files at the target.
3. You might not be able to use verification since the target directory contents won't always match the created hash sums.
It might be much better using a real sync instead of just copy.
Microsoft update catalog: http://catalog.update.microsoft.com/v7/site/
Windows Install media creator: https://support.microsoft.com/en-us/help/15088/windows-create-installation-media