Are usernames a valid candidate for a primary key?
NO, you should use username only as a unique key/constant. Username can be changed, just and example: it may be a trademark and the owner requests you to drop it.
It is a good candidate in some sense, however you have to consider if you really want to do it. For instance, you have a user with a certain user name, then the user gets "deleted" (or marked as deleted). So, there is actually no reason for not allowing to create another user with the same username, but it is already "taken", since it's a primary key.
From a customer standpoint, you bet. I don't want my username to be DWong1145.
A big disadvantage of having a string as primary key, regardless of whether it's a username or something else, is that all foreign key columns referencing the table will also have to be strings, which is both slower and wastes more space.