I recently built a ROCK using an 8i3BEH in an Akasa Plato fanless case. It’s fast (as long as Roon isn’t having its “search issue”) and, in the fanless case, silent.
The standard NUC is not quiet. It was louder than both an old Dell i5 laptop AND a Synology NAS which I was using in the same room previously to run Roon store/core. The NUC was noisy even at idle and the fan is an annoyingly high pitch. If you’re using it outside the listening room AND in a space where you don’t need things to be quiet (like I did in my office) then you’ll be OK with the standard NUC, but I’d say there’s most certainly a reason why the Roon team designed the Nucleus to be fanless.
The transfer into a fanless case was pretty easy as long as you’re careful, follow the instructions, and use common sense/ask for help on the forum for anything you’re not sure about. Check out my thread on it here if you like, especially re: the difference in BIOS procedure between the Roon ROCK install KB and the actual BIOS in the newer 8xx series.