I don't really consider NUCs or other off-the-shelf commodity x86/amd64 mini PCs to be "embedded." Especially if they're running an off-the-shelf commodity OS

I don't really consider NUCs or other off-the-shelf commodity x86/amd64 mini PCs to be "embedded." Especially if they're running an off-the-shelf commodity OS that can throw a BSOD. That's not some custom distro that cannot be updated; whether they actually care enough to update it is a different matter altogether. You totally can pick a supported distro with LTS kernels, and keep it up to date. I'm writing this as someone who's used Shuttle's fanless mini PCs (designed for PoS/kiosk use) as desktop & server hardware, all with proper updates. And I work for a company that does actual, custom embedded hardware. I've made a billboard too. None of the actual embedded hardware (almost exclusively ARM) I've used is SMT-capable. Even among off-the-shelf amd64 solutions, it's common for people to stretch the penny and buy a celeron/pentium without hyperthreading. And, fwiw, I've never witnessed a BSOD on a billboard in person.