I have an Intel NUC D34010WYK w/ Intel Core i3-4010U Processor from around 2014 that's sitting unused (used to be my HTPC).
Would OpenWRT be able to run on it? Is it a good idea to run it as a one-armed router and expect it to NAT full 1/1Gbps internet?
I know this is the OpenWRT forum, but would I be better off running pfSense on it or even just plain Linux, e.g. Ubuntu Server or some other distro and configure it all by hand?
I have an incoming Ubiquiti EdgeRouter ER-12 but it just occurred to me that maybe I don't really need it. Or maybe it's better than the NUC and then sell the NUC to fund the ER purchase.
That depends on the idle power consumption (which should be fine for this device, but you'd need to measure it).
The CPU should be able to do that (easily), but not symmetric 1 GBit/s with a single ethernet car. All packets have to come in from WAN, pass through the CPU and go out to LAN again - in theory this cuts your throughput in half (plus overhead), in practice it should be a little better (as it's unlikely that you'll have symmetric traffic at >500 MBit/s all the time). However, your device comes with two mini-PCIe slots (there are rtl8168 based 1000BASE-T ethernet card in mini-pcie form factor, with the rj-45 port 'on a leash' for <15 USD).
OpenWrt is predominantly tuned to being a router distro, as long as you don't expect too much beyond that usage scenario (in particular externally facing server dæmons and their security implications, which would require timely in-place upgrades), OpenWrt will do the job - as well as it would on more common (plastic-) routers. With its linux base, OpenWrt will provide wider hardware compatibility over pfsense (if you want to go this route nevertheless, I'd look at opnsense though), the webinterface and router targetted configuration will easier to use than a generic desktop-/ server linux distribution. Both have their own advantages over OpenWrt as well (e.g. in-place upgrades at runtime) and being less targetted at low system requirements (maybe more featureful additional package configurations), but neither should be overly relevant for dedicated router (with maybe VPN and other 'tradtional' router extensions on top) uses (but can become crucial if you want to abuse the hardware to serve additional ressources as well).
With a second (mini-)PCIe ethernet card (maybe USB 3.0, if you can't avoid it), the haswell i3 should cope with symmetric 1 GBit/s routing easily (additional costs to make it two-legged, around 20 USD); power requirements should be roughly comparable between both devices. The advantage would be wider OS compatibility (e.g. OpenWrt) and the option for additional 'abuse' (less common ~services). What the ER-12 can offer beyond a NUC, is mostly its internal managed switch (but due to the port design, you'd still limited to 1 GBit/s per port; it does not seems to offer 10 GBit/s support (or 2.5GBASE-T/ 5GBASE-T)), but you're basically limited to the OEM firmware (it's one of the last mips based cavium SOCs, third party support would be possible, but is rather limited - and exotic in any case).
The decision is yours, eventually - and you should do your homework first (measuring idle power consumption, testing the setup with an additional ethernet card, making up your mind what you actually need/ want from it).
You are right about 1/1Gbps symmetric on a single card, you can do 1Gbps total up+down. In practice I'm not even sure what to do with all that bandwidth, I "upgraded" to it from 100/100 because believe it or not it was cheaper on a special promotion.
The other concern I have with a one-armed solution is that I need the WAN cable to come into the managed switch directly (on its own VLAN) but not sure how secure something like a consumer grade managed switch (e.g. Zyxel GS1900-16) is to expose to the Internet. I understand that the management VLAN would not be on the WAN VLAN, but who knows what bugs these things have.
You're likely better off running pfsense as it's utilize the hardware much better (provide more funtionality, better overviews, more reliable and frequent updates) however I'd highly recommend you to get dual Intel NICs which can be a bit pricy if you're looking at mini PCIe form factor but I would highly recommend those over Realtek.