One that NAS in general has some trouble with, and that FreeNAS isn't able to address particularly easily at the moment, because it is based on ZFS, which means "big hungry server."
I've been operating relatively-low-wattage NAS units for years. Units such as the iomega StorCenter IX2-DL take a handful of (~6-8) watts to power the system, but didn't offer substantial performance (Marvell 6282, 256MB, etc). The problem we needed to solve was bottom tier shared storage, and the fact of it is, four of those and 2TB drives gave us 6TB of RAID1 iSCSI storage, which cost a total of maybe $1200, or $200/TB. That was ~2011-2012.
Those are getting long in tooth and FreeNAS isn't really competitive pricewise, so I'm in the midst of replacing those with some Synology DS416Slim units. They're still not *great* performers, but they're pretty reasonable, plus with four drive bays, plus *tiny*:
which being a SoC based NAS is also very low power. Putting two crap-grade 960GB SSD's and two Spinpoint M9T's in each one, so we'll have four of them with 8TB of HDD and 4TB of SSD, and total project cost is around $3500, or around $300/TB. Which is more expensive, but, hey, SSD, more space, redundant ethernet, faster, ..... not bad.
By way of comparison, the high performance FreeNAS based ZFS VM filer was costing maybe $7K to deliver 7TB of usable space which will absolutely decimate those little guys, but it is delivering half the space at twice the cost and three times the energy consumption. I had been hoping to avoid another round of SoC based NAS units but things still aren't quite "there" for FreeNAS.
Each of these solutions has their own benefits and downsides, and if energy consumption is a primary consideration, one of the smaller SoC based devices might be a better choice, just because there's no need for lots of RAM and lots of CPU there, and there are companies specializing in building those. FreeNAS is built out of standard server components.
Haven't seen that to be exactly true. Our datacenter footprint has shrunk over the years while the complexity of the environment has increased. Thanks, virtualization. But in general the demand for data center space seems to be growing even with things like cloud being an an alternative-of-sorts. I sometimes wander around Equinix and kinda wish I had a little genie that could tell me what a given bit of gear was contributing to the world.