NUCs (specifically Intel 8th?-gen or later) are pretty much ideal for serving Jellyfin because the Intel integrated graphics can do video transcoding and the software is actually not very demanding otherwise, so the low-power CPUs are fine.

If you were buying hardware specifically for Jellyfin (i.e. didn't want to cobble together something used), I'd suggest an N100 or N150-based NAS mini-PC like this: https://www.bee-link.com/products/beelink-me-mini-n150