Actually, I think you missed a significant point. Google's OS is a Linux-based system, but with some very significant modifications. The biggest one is that user data is NOT stored on a single server; it's stored across multiple servers. So even if a hacker were able to compromise *A* server, that wouldn't give them access to very much of your users' data. They'd have scraps; bits and pieces of information, that are likely pretty useless on their own. To that end, their solution may actually be MORE secure than hosting it on your own in-house server. As for how that compares to Office 365, I don't really know.
Keep Up with TechRepublic