If you look at pfsense/OPNsense hardware recommendations, it's almost all using chips like the Intel N5105 (10W TDP, though admittedly "TDP" is itself a messy term) or J4125 (also 10W TDP). Using ARM hardware is asked a lot in the community forums, and it's one of those questions that will get you a flamed for not checking Google first. The power usage benefits for switching to ARM just aren't there.

There is the Netgate 1100, which runs ARM on a proprietary build of pfsense. The community has largely ignored it in favor of Intel chips. There isn't much of a price advantage, and the performance is lackluster.

That said, there's lots that you can do with a sub-10W chip, and x86 has nothing modern there.

Personally, I cobbled together an OPNsense firewall out of some old desktop parts I had on hand. Power usage is a bit higher, but not so much that I care. I would like a more viable high-end ARM option, though, just because I don't want x86 to be the only option.