I have a site with a lot of items in the menu.
When I created a custom menu - the categories are not consistently displayed as the parent/child hierarchy. Sometimes there are a few child items displayed hierarchically - and then most are not - sometimes they are scattered all over the place and I have to find each item category independently.
And - when I used the "search" function to try to narrow down the options listed - it does not bring up all the categories, only some of them.
it is in "coming soon" mode - but you can see the menu on this log-in page ( http://supplymyschool.com.au/wp-login.php )
I have finished building the menu now - but I'm passing it of to a client who will want to add categories to the site and the menu - and I can see it being quite a frustrating process.