The Gentoo manual page about it is filled with warnings that this is a tool under development.In the meantime, Gentoo rather encourages you to update the whole system.
The easiest way to describe the benefits of this is by comparison to a normal Free BSD server and the installation process. Suddenly, X11 becomes a dependency and you find yourself hitting Ctrl-C rapidly! As we have mentioned in this blog before, there is an excellent forum for Gentoo users.
This way you can be fairly sure it’ll work even after a reboot. It also knows what steps need to be taken to fix a given security issue.
Once you’ve verified that everything works, switch to the new system. I really like this development, but I understand that this command is not considered production ready.
Unfortunately Gentoo encourages you to update software on a frequent basis, just for the sake of updating. Gentoo is rather a moving target where emerge will forever cause your system to approach the cutting edge.
From the Gentoo handbook: If all you’re concerned with is keeping your web server up, what you usually want to do is to set up a stable system and then forget about it.
Search for updating gentoo:
This is in fact exactly what happened to me, despite a substantial time spent updating /etc files.