|BUG: Updating (not setting) Registry causes invalid values|
Windows Installer handles updates to normal registry string values OK (using the "[~];" type syntax) but I have seen it fail when updating the Windows device driver search path "DevicePath" (an expanding string).
Note that the "DevicePath" value may or may not contain unicode, it frequently does (and thats how Windows creates it). You can't tell which you have unless you save as "REGEDIT4" or other text format and if every 2nd character is a "00" then its unicode. Its possible that the Windows Installer problem is more generic and applies to all types if unicode (that is the issue might be with unicode strings of any type).
See the "Registry Appending" section for an example of a workaround which uses a custom action.