Used Chromeboxes are inexpensive, and pretty close to a NUC other than the BIOS. But, people have figured out how to get them to boot Linux, Windows, etc, now. There's also pretty nice used USFF boxes from Lenovo, HP and Dell: https://www.servethehome.com/introducing-project-tinyminimic... The older ones, like a Lenovo M72 tiny, can be really inexpensive.

To me, they aren't always a better fit than an Rpi, but they often are.