It doesn't seem to block all special characters though. I was able to put square brackets in the title.
EDIT: for some reason it allowed single quotes when editing my post. Maybe it was just the '&' that was causing issues.
EDIT2: '&' doesn't seem to cause issues in posts either. It may just be effecting the bio and nothing else.
EDIT3: I'm getting an error that says: SyntaxError: JSON.parse: unexpected character at line 1 column 1 of the JSON data when trying to upload an avatar. I'm removing [solved] from the title for now. I'm also going to submit an issue on github later.