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My work place runs Gentoo on production servers too... and I myself runs a Gentoo machine for my daily analysis tasks. We have excellent runtime and we haven't run into a problem yet.
The point is, anyone that runs emerge -u world daily without checking is just asking for trouble. How many people would actually let Ubuntu, Debian, or RHWS auto-update to the next major release automatically on a production server? People would say that those people are idiots.
For me, I never emerge any big packages until it's released for 3 days after stable (let the early adopters install and found the issues for me). For kernels, aside from GLSA advisories, I only wait until -r3 before installing. For critical packages like glibc I wait at least a week before installing.
I always take a quick look before emerges. With the older hardware I run, a "emerge -u world" would probably kill my nvidia-driver for new every new update nvidia put out(for example, nvidia recently decided to change the 1.0.87xx versioning convention into the 87.xx conversion for no reason). So, Gentoo, like other distributions, requires a bit of care when updating.. just that the care is spread out over time rather than one big honking update.