Having over 15 gateways in the field and having written and released a packet forwarder I am aware of the (low) load it imposes on the CPU. However for forwarders not using the ancient UDP protocol the memory footprint is relatively large for the mostly limited amount of memory in the average openwrt device. So the basic router functions and wireless (WiFi and 3G) backhaul won’t be a problem but something like SDN might well be something that tips the scales the wrong way.
I assume you’ve based your hardware on something with a decent amount of ram and flash and not the 16 MB flash the LG308 has. For that device with the stock firmware that’s sufficient flash but It is not a lot when you want to add functionality. At least not for the average programmer these days, they’re used to gigabytes not megabytes.
(I happily use controllers with a couple of KB. I started programming on a CBM3008 with a whopping 8K ram and tapes for permanent storage and a ZX81 where the 1 KB of ram was shared between the program and display memory)