I dunno - it has never failed me. I used it today when I installed Windows Media Player 11 (Beta 2). I found that WMP11 could not play my MIDI files because it could not deal with the SoundMax synth, and was looking for a file that did not exist (synthcore11.exe.) So, I just ran System Restore and it removed ll and put the system back to 10. No problem - only took a few minutes.
Sometimes other things cause programs or data to be moved or changed AFTER a restore Point is established. Then it can't restore to that date.
For me it is a valuable tool - it does a little more than just change the registry.