Sometime between Evolution 2.24 and Evolution 2.28, search folders on unread messages stopped being very useful. This is because when you start reading the message, the message is marked as read, and it disappears from the search folder before you have time to read it.

It used to be that in this case the search folder only updated its contents when you expunged it, for precisely this reason. (I know this because I was involved in the programming of the Evolution 1.0 mailer.) This allowed you to have a folder with only unread messages in it that you could read and periodically refresh throughout the day.

Furthermore, as of at least Evolution 2.24, when you expunge a search folder that looks for unread messages, it doesn't seem to pick up any new unread messages that have arrived. As far as I can tell, such folders never pick up new unread messages without exiting and restarting the program.