Well NUC is kind of generic since you can get them with up to an i7 all the way down to a Celeron in them.
That being said though I run LB/BB on a large faction of PC's from an i7 with 16GB of RAM and a GTX 960 all the way down to a Insignia tablet with 1GB of RAM and an Atom CPU and while its much smoother on the desktop it can be tweaked to work on lower end hardware (still not the greatest but workable). If your emulation goals are modest ie only NES and SNES you could probably get away with an i3 but I would suggest at least an i5