Bazel is there becoming ever more complex and unwieldy in an attempt to provide supposed reproducibility - taking control of the provision of ever more of a project's dependencies (in often very janky ways). But to Nix people it's clear that what people are actually doing here is slowly building a linux/software distribution around their project, but in a very ad-hoc and unmaintainable way. And bazel projects will continue to grow in that direction because until you have control of the whole dependency stack (down to the kernel), you're going to struggle to get robust reproducibility.
I don't think many Nix people would suggest actually using Nix as the build system, but probably to use a comparatively simple cmake/meson/whatever build-system and use Nix to provide dependencies for it in a reproducible and manageable way.