What matters most when choosing a new NUC for Roon Rock?

Completely agree.
What is in the guide is misleading, as this recommendation will never work with large libraries in local storage.
We failed badly following this.

I thought Roon Rock does not work beyond NUC 10, but posters here are recommending Nuc 11 and 12. Does Rock now support the newer NUCs?

Yes, worthwhile checking out their recommended NUCs list. 12s don’t support HDMI out yet and there are people that have used ROCK successful on a NUC 13, though no official support for them yet.

You can always find the official list here: