I worked on an OpenWrt-derived product for some years and I found their build system quite good. If I remember correctly it's based on buildroot but they went separate ways somewhere in the past?

In any case, I found it quite easy to add new packages, update existing ones, rebuild as needed, add support for new devices, image generation, ... Also, saving the build config and used package versions to rebuild the same image later was easy. Documentation could be better (obviously...) but there are plenty examples and one could always ask advice on IRC or mailing list.

I don't have experience with Buildroot, Yocto, OpenEmbedded, ... but at least the OpenWrt build system was much better than the homegrown stuff I worked with before...

So if you're looking to build software for a Linux-based device that has some networking functionality then you might want to look at OpenWrt. If not for OpenWrt as such then perhaps just the build system.