Initially, chromium-bin was just a snapshot compiled by the upstream's buildbot, and it had many problems on Gentoo, like bug #303080.
At some point I converted the package to use a binary built on a Gentoo system, to avoid the libjpeg mismatch. However, because arch (stable) build environment was used, it was incompatible with the ~arch systems, and as a workaround dependencies had to be adjusted in an ugly way (bug #347175).
Because chromium-bin was never stabilized, I thought - why not just switch to ~arch build environment? Well, that also has problems. First, arch (stable) users who unmask chromium-bin hit bug #348738. And, as ~arch is not really stable, I hit bug #348587 when trying to compile a more recent version of chromium.
I'm not sure what the end result will be, but I'm considering going back to stable (arch) build environment, and stabilizing chromium-bin, so that stable users don't need to unmask anything. Of course it's very likely to break chromium-bin on ~arch...
A possible workaround is using more bundled libraries for chromium-bin, like ICU. It would prevent issues like bug #347175, but I'm not sure if I really want to do that.
Oh, and in theory we can have different builds for arch and ~arch. However, to do that we may need more people in the team.
If you're interested in helping with any of the above, please contact the Chromium in Gentoo team.