In Nixpkgs there are quite a few generated packages, some of which cannot be built. I'm maintaining R packages (from CRAN and Bioconductor) in Guix and I see that the equivalent packages in Nixpkgs do not accommodate some of their quirks, so they cannot actually be built.
Another example where Nixpkgs does not build packages from source is TeX Live. The SVN repository of TeX Live contains countless generated files, and all of the TeX Live packages in Nixpkgs merely copy the generated files from SVN instead of building them from source.
Java packages in Nixpkgs often seem to include pre-built jars whereas in Guix we're going to great lengths to build everything from source. This comes at a cost, of course: Guix has fever Java packages because of that, but those that it has are actually built from source.
Of course, I'm biased towards Guix, but I must say that I'm very fond of the level of integration that Guix achieves. When I look at Nix I see a bunch of seemingly separately developed tools that are written in a mix of languages (with Shell code being rather common). In Guix all tools are written in Scheme, use the same API, behave similarly, etc. It just feels much more polished to me.
That said, there is no animosity between these communities, and I think we would all be better off avoiding the mental pitfall of competition; we work on the same problems with different values and thus our approach differs. This leads to diversity rather than duplication of work.
Here are some more things I wrote comparing Nix and Guix in the past: https://news.ycombinator.com/item?id=19807042