I can’t speak for the lan1641 card, but I have two net5501 boxes. One has been running Astlinux for over a year now. The second is a month or so old and running Zeroshell. On the Zeroshell box I have a generic 1 gig Ethernet adaptor installed to give me a total of 5 ports (2 WAN, 3 LAN) and am running three separate LAN segments. Works pretty well. Quiet, low power usage. So far no hardware issues at all on either box.
The CPU loading is higher on the Zeroshell box (0.04 0.12 0.09) than on the Astlinux box (0.00, 0.00, 0.00). I don’t know if adding more Ethernet ports on Zeroshell would affect CPU usage or not.
My Zeroshell box replaced an old Xincom 502 dual WAN router. On my primary provider the Xincom topped out at 20 Mbps download. Same link, same time of day with the net5501 running Zeroshell gets me 30 Mbps. That is all the ISP claims is available so I don’t know if it will handle higher speeds than that. I am not running VPNs, I am sure that would slow things down.
One downside of more Ethernet interfaces is that the graphics unlock codes are based on the MAC address. I think (don’t know because I’m not using VLANS) that if you ran all your LANs as VLANs off one interface you’d only need one unlock code for the pretty pictures. Putting them on separate adaptors would mean you need codes for each. I only bothered getting unlock codes from Fluvio for my two WAN interfaces.