Someone from the support team will need to confirm the special characters supported but in testing it seemed to work with everything I threw at it.
I did have an issue with special characters in that PMP needs to be told what keyboard to use when running in RDP sessions which highlights the fact that if you are a global company you can expect some fun when managing passwords with special characters in as these are the keys that get moved all over the keyboard depending on which location you are in.
You might want to think about excluding ones that you know will cause issues like the @ key which is in a different location on a US keyboard to a UK keyboard.