I know a little.
Things that bring down all the sites are more related to change to the code, not stuff in the database. All blogs/sites are served through one copy of the files. If you make a change to a file, it affects every single blog. make a mistake, you've brought down all of them.
But that's more dealing with things like "don't edit production code unless you know what you're doing". ;)
If you're running a network for others, they do not have the privileges to edit themes or plugins. They can't install them either. Only the Super Admin can.
If you're interested in security, then you're probably vetting all themes and plugins before you add them to your site.
So, that takes care of anyone adding any code that can eventually access the database(s). (You will need to scale the dbs when you get bigger).
really though, mu has ben more secure that wp with the locking down of things. Your first line of defense is *always* securing the box the install is on.
The recent round of hacks targeted wordpress, but did not have anything to do with how secure WP is. Keep your OS up to date, for instance. Don't use FTP, but use sftp instead. Keep secure passwords to your server (never log in as root, log in as a user, then su).
Short version, in my understanding, if someone's breached your database, they've already breached the box.