At least WordPress' updating system is good...
One of the big selling points for me about WordPress is that it is *very* easy to update (though I'd like to see more thought put into easy rollback other than "restore the Web/DB dumps you took before the updates"). The recent 4.2.3 and 4.2.4 security releases actually got applied "automatically", generating an e-mail to the site admin to inform them of this. 4.2.3 broke the Types/Views plugins for a lot of sites (and it took about a week before Toolset released a fix, though it was possible to workaround the issue or indeed manually rollback, which is why I think easy rollback would be a nice WP feature to add).
Perhaps the weakest issue with WordPress updates is that there is usually no way to specify an auto-update for all your plugins and themes (or indeed for a major WP core release either), though you can homebrew something up with the WP-CLI tool.
Easy updating, to me, is a *major* selling point - some CMS'es I've seen are utterly appalling when it comes to updates, often requiring days (I'm not kidding) of work to upgrade them.