Wow, but they are not special, except me
Did you see the username like me ? i see only Ba™aN.
It is risky in some cases, I saw people reported / getting negative trust for being imposters. For example : theymos - theym0s.
Well, using special characters is easier to distinguish but i do not recommend it.
For the A-Z... I think people tend to register usernames and keep it just like registering domain with three or four letters in order to sell it later. I remember people even registering famous companies names or presidents names