I've been using single Wordpress installation for my site since 2012/09/06. To make it clear, it is the same installation I've installed way back then.
Never had any security issues with it, but I don't want to push my luck now ;-)
Also never had any major breaking changes. Except some weird display issues with the previous theme I was using after update some time ago. Nothing major, but was time to consider new team anyways as the old one wasn't maintained for quite some time then.
social.tomica.me is one server in the network