Define "broke" :)

Updating to 4.0 would definitely have destroyed post-editing functionality for one of my then-client's site, since they were at the time using the old Google Maps (which at the time created a conflict with the visual editor and adding images), and it took a long time for that plugin to be updated to deal with the conflict. That delay would have been unsatisfactory situation for that client, since they did a lot of content updates, and did them themselves with less than tech savvy folks.

So while it's true that particular conflict only affected a small number of sites, plus the fact that a list of conflicting plugins is maintained every time a new WP release comes out seems to suggest that things will break, and if they break during an auto update in the middle of the night with no one attending the site or monitoring the results, Bad Things Might Happen (tm).

Thus, no automatic updates for any sites for clients I do maintenance for, and not even on my own sites (except on testing platform sites).