I think the problem is not with the actual IP routing part, as that is relatively straightforward, but with all the other value-added "features" that get crammed into the same and often underpowered hardware. At the least, a consumer WiFi router will contain an IP router, a NAT, and an AP, and it's the NAT that probably contributes a lot to the connectivity failures people are seeing that go away when the "router" is rebooted --- NAT tables filling up due to many half-open or lingering connections that didn't get closed properly. The aggressive nature of many applications' connection usage (e.g. "why does my router stop responding when I use torrents?") doesn't help either.