while recently trying to remove a server that was being decommissioned, we could not find the customer that WHMCS believed to be assigned to the server.
In the end, we had to perform a sql query against the database to determine which account was still left on the server - it was an account that was cancelled/terminated, but as it still shows on the server - despite showing 0% usage, it prevented server removal.
I would suggest a list of assigned accounts be accessible in the servers list view - perhaps expand the server usage to include assigned / active / out of total allowed.
Make this count of usage section a link to customer plans assigned to the server - I'd further suggest that the list be in sections based on status.
One past piece - make the accounts able to be selected with a checkbox and actions such as email owner etc , terminate, cancel etc - so that mass communications or cancellation can be done on a server basis in a few clicks.