We've been doing this with BU Linux for about seven years. In that time (minus one exceptional situation, which wasn't that severe), the only Linux systems broken into were those a) running a different distribution or b) who had disabled the automatic updates. We had a couple of incidents where QA failures caused issues, but the number and severity of those occurrences pales compared to cases a) and b) (where system compromise appears to be almost inevitable).
Admittedly, this is with a installed base of 1200 systems or so rather than millions, but it's also in a very hostile security environment. I think it makes a good case study.
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