Simply NUC just announced the Sequoia, a small form-factor NUC-like device with AMD internals. It can be configured with either a Ryzen Embedded V1807B or V1605B and is designed for industrial use cases. But with prices starting at £445, it might just be a good fit for your HTPC setup, too.
https://www.notebookcheck.net/Simply-NUC-s-Ryzen-powered-Sequoia-doesn-t-have-Intel-inside.443193.0.html