FWIW I have an old router from my ISP, I put OpenWRT on it, repurposed it for HomePass with
this, and it seems to be running fine. I just close the console and get up to 10 hits, every 5-10 minutes, every time.
I believe the main problem with HomePass being unreliable is due to the variety of methods and hardware people use. Consumer hardware is not expected to be able to change its MAC address. Software and drivers have to resort to hacks and workarounds to do it, and they are often buggy. The OS/antivirus/firewall may react badly to what it sees as suspicious behavior. And then some wireless adapters are just picky and don't even work reliably under normal conditions.
On top of that, for Homepass to work, the 3DS is supposed to be able to connect to internet through the Homepass, even if it doesn't have any wireless network configured (it might even download firmware updates through it, I've seen it happen). So an ad-hoc wifi hotspot isn't enough, it must also be bridged.
edit: bridged
and without password. So security could also be an issue.