On Sun, Mar 03, 2019 at 11:06:32AM -0500, Mark H Weaver wrote:
> I don't know about berlin, but hydra.gnu.org has *never* successfully
> built ungoogled-chromium.  So far, it has made 3 attempts on
> x86_64-linux and 5 attempts on i686-linux:

[...]

> If the same is true on berlin.guixsd.org, that would explain the lack of
> binary substitutes.

Berlin did build it, soon after the package was first added. I used the
substitute on my machine which lacks the capacity build the package.