Really? If someone asked you if exclamation marks in usernames would cause problems before you were aware of this, would you have said yes? It's a very common special character, it's not like it's a control character or some obscure unicode thing. Besides, it's a common thing to add to usernames outside of services where you use your real name.
Absolutely, I would have said yes to the question "do exclamation marks have the potential to cause problems in some piece of software or other, the likelihood going way up with the number and the obscurity of software you use on this machine".
