3. Embedded Flash implementation (which doesn't exist on mobile anyway).
4. Google API keys.
If what you care about is security auditability, that's pretty good. If you care about running only open source software, that's going to be very hard to do in the Android/Google-Play ecosystem.
Main advertisement? I just went to android.com and developer.android.com; android.com advertises "Google built in" and lots of platforms, with a very small link to AOSP at the bottom of the page; developer.android.com has an AOSP link buried in its menus.
the 1st block is about the nexus one (market as open, but not on this page) and look! the second item on the page reads "Access to the entire platform source and information on how to contribute."
guess they forgot an asterix there saying that the "entire platform" means some of the platform.
I believe only Chrome has built-in PDF viewing too, which can be nice. The page you linked has people saying there are Chromium plugins for it, or you can install a dedicated PDF viewer and it will probably embed itself in the browser when downloading PDFs.
tl;dr: Chromium is Chrome minus:
1. Crash/usage reporting to Google.
2. Proprietary video format support
3. Embedded Flash implementation (which doesn't exist on mobile anyway).
4. Google API keys.
If what you care about is security auditability, that's pretty good. If you care about running only open source software, that's going to be very hard to do in the Android/Google-Play ecosystem.