Just to add to the low power success stories.
I have been using the following build for two years and am very happy with it:
ASRock J5005-ITX Motherboard
G.SKILL Ripjaws Series 16GB (2 x 8G) DDR4
Silverstone 300W SFX 80 PSU
FebSmart PCIE 3.0 SATA III ASM1064 Controller
WD Blue 3D NAND 250GB SSD
6x WD Red Plus HDD (ranging from 8gb to 14gb)
I don't really know if there's an issue with PCI lanes. I have my drives spin down when unused and doubt I ever use more than 2 or 3 drives at a time except for perhaps during parity check, but the parity check won't use the cache so it's still 6 drives max.
I also use a few Docker images including OctoPrint, Deluge, Ubuntu, and Plex with QuickSync hardware acceleration for transcoding. The speed won't blow you away but it's been surprisingly good for everything I've wanted to do with it.
I hadn't measured it before today, but power consumption at idle ranges between 20W (disks spun down, Plex and Ubuntu still running) and 60W (all disks spun up). Maybe add an extra watt for the top exhaust rack fan which is separately powered. I'm really surprised that it's this good. I could probably improve it by messing with power states or getting a different PSU.
Intel is still making embedded chips in the Pentium and Celeron lines but they all seem to go into netbooks and chromebooks; I haven't seen an all-in-one motherboard with one. I think maybe the J6412/J6413/N6415 is the top of the line for ultra low power.