I'm in the cheap azz category here. I use 2012 Mac Mini Core I7 something/ssd (free from last work) as my Security camera video recording box or NVR is the fancy term. I record 5x 720p cameras around the house and it's hauling azz just fine.
For my actual htpc (client) I still use atom/nvidia based very tiny asus pc. recently swapped 2.5" disk for ssd. it helped a bit. It's fast enough for local/streaming 1080p and bitstream hd audio to avr. The key is not the processor, but video card acceleration which does heavy lifting.
Running recent build of OpenElec. I agree that NUC is more than enough for HTPC. If asked I'd say modern Pentium class cpu is more than enough.
Thing to worry about/Exceptions are: 10 bit videos (mostly anime), H265/AVC and 4k. For now most of these are edge usage cases AFAIC