The learning curve is a little steeper than more consumer stuff but it is by no means beyond a person who is capable of using OpenWRT and the docs and forum support are better than 99.9% of open source projects I have seen over the past 25 years.
So that's 10 kWh/mo for the PC vs 4 kWh/mo for the OpenWrt One.
Spending hundreds on new hw to save $20 a year in power cost.
I don't think you're quite right on this, or at least you're imagining using something inappropriate when the comparison here involves buying something new right? So it's not "OpenWrt One" vs "whatever you happen to have in your closet" but "OpenWrt One (~$110-130)" vs "whatever can be bought used for $110-130, if you have nothing appropriate". And while they won't go to near-zero like some ARM stuff might, idle power for PCs improved a ton after around the 2013 era. There are lots and lots of small systems available for equivalent prices on Ebay or the like made since then (like Intel NUCs or various other mini PCs) that will idle around 4-10W. Like to take something in the same price range as this OpenWrt One, I regularly see 7th gen era NUCs going for <$140. An i5-7260U will have single threaded performance about the same as the MediaTek in this unit and multi-thread close, but will also generally have 8-16 GB of RAM and often a 250-500GB NVMe drive as well. It'll probably have only one native ethernet, but USB or TB adapters work fine with Linux & FreeBSD at this point.
There's definitely a question of values and exactly what you're trying to focus on, but there are a lot of niceties in having lots of RAM on tap and extremely standard fallbacks to interface with a system, back it up, etc.
>or an OpenWrt One which will possibly also have newer, faster WiFi standards (WiFi 6)
If you want an AIO style device that's definitely a consideration, though again USB WiFi dongles are a thing too. But regardless of router choice, for someone considering going beyond what their ISP offers at all I think it's usually well worth spending the $50-80 to get a dedicated wireless access point. It'll make a major difference in real world performance in most spaces I've seen to just physically have a unit in an ideal spot (on a ceiling or high up on a wall, away from metal). Aesthetically the clean disks or rectangles those tend to have also blend well and mean that various boxes can be tucked away. And of course you get to upgrade networking bits separately from your router.
Anyway, definitely good there are multiple approaches, this is an area of life where people can have very different needs driven by very different physical environments and "stakeholders" (like significant others). But I think OPNsense (or other bog-standard-PC FOSS alternatives like VyOS) can be competitive even in TCO, depending on how much value you place on pushing your networking stack and what else you have going on (like solar power).
It was the most incredibly hassle free router I've ever had.
Hopefully, the OpenWrt One is like that. But if not, I'm going to go back to killing the planet with my loud ass beater PC.
Beside the potential performance and environmental issues the other big downsides tend to include firmware availability - either because download from the site requires a login on the vendor's site or, increasingly commonly, the gear has hit LDOS and images just aren't posted. Obviously there are other "unofficial" places for such images, but the risk/legality are a whole other (potentially serious) question.
There's an additional issue mapping the requirements of home networking to enterprise gear: Ethernet switches are lousy firewalls (little or no NAT, primitive built-in security, DLNA/mDNS and friends aren't really sane options, etc). Finally, even at "1/5" the price the gear may still be quite a bit more expensive than other options. And if it's not expensive, it's usually because nobody wants it any more because of the issues mentioned above (being near- or beyond- LDOS).
FWIW this is from someone who literally built a commercial-class machine room in his house with dedicated AC, subpanels, commercial UPS, etc for data center class Ethernet, Fibre Channel and Infiniband switching as well as carrier-grade routers and still runs "enterprise" grade WLAN and switching and can lay hands on as much as I could reasonably want without too much drama or cost... Going down this road can absolutely be amazing if you either have a.) the background to properly source and run the hardware/software or b.) have a driving desire to learn how to do so or c.) have some very atypical requirements for home networking. Otherwise it tends to not be something to be done lightly.
My latest 3 home routers have been mikrotik, mikrotik and juniper.
And the Juniper is still in there, just running as a switch. The old juniper srx sucks at uPNP but is otherwise still a fantastic router if I wanted to swap back to it.
Middle tier mikrotik took a power surge and lost 50% of its ports. It still routed just enough for me to get a replacement.
Latest tik is doing great.
I dont see what the One offers me at all tbh.