Is there nothing better than Yocto?

Buildroot has been used for a lot of Linux-based space missions. When I talked to people doing Linux-based cubesats at the SmallSat conference a few years ago, many of them had not heard of Yocto. A lot of projects get their kernel and distro from a satellite bus supplier (which usually includes the flight computer), and don't build the kernel themselves. In my experience, fiddling with details of the OS is a low priority task, relative to the custom development that is done for whatever payload the mission is carrying.