Chromium OS (and Google's commercial version Chrome OS) is designed to work primarily with web applications. Aimed at users who spend most of their computer time on the Web, the only native applications on Chrome OS are a browser, media player and file manager. The user interface takes a minimalist approach and consists almost entirely of the web browser.
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Pro Safe and requires little to no maintenant
Since you only have the browser, you cannot install applications (outside Chrome Apps which are decently protected by their limited API and the Chrome Web Store). This means no harmful virus, and also nearly no way you can screw your system. Everything will just work.
Pro Minimalistic interface
Chromium's system UI uses as little screen space as possible by combining apps and standard web pages into a minimal tab strip: While existing operating systems have web tabs and native applications in two separate strips, Chromium OS combines these, giving access to everything from one strip. The tab is the equivalent of a desktop application's title bar; the frame containing the tabs is a simple mechanism for managing sets of those applications and pages.
Because Chromium OS is designed for users who spend most of their computer time on the Web, it is intended for use in computers with little local storage and fast boot-up times.
Possibly harms your privacy.
Con No auto-upgrade (unlike Chrome OS)
Chrome OS auto-upgrade the system, but Chromium OS does not. It's possible that they're thinking about adding that feature from the design doc.