So Go was designed for large software systems, and maybe it's really good at those things. Or maybe not, because nobody's written one yet. The article says so: "We don't have enough experience yet, especially with big programs." So Go is unproven in this way.
Speculatively, my guess is that it won't be able to "at scale" its current state. For example, Go errors out with unused variables, imports, or indeed for all warnings. Imagine trying to make a piece of software like WebKit or LLVM compile on all of the platforms it supports, with all of the compilers they support, without introducing a single warning (or extraneous header or unused variable) on any of them. It would be a maintenance nightmare. You need some flexibility - demanding perfect hygiene for every cell on an NxM matrix of supported platforms is infeasible.
In the other sense of scale - large data sets - Go apparently works well enough to be involved in YouTube. That's cool and is a credit to it.