Not if you need to communicate with other user space services, such as:

* systray icons/application indicators

* newer fontconfig versions support new config options which results in fonts being broken for apps that use older versions

* I think there is some issue around glibc locales handling if you use a different glibc

* systemd/logind/... DBus apis

Those were just some external interfaces that came to my mind. I'm sure there are many more.