Cost, both from power consumption and purchase. Hard to compete in those fronts with a mikrotik or ubiquity router doing 1Gb/s through hardware offload with a tiny MIPS or ARM chip.

One thing I've seen some people run into is if their ISP insists on PPPoE, inbound packets will tend to stick to a single RX queue, which lends to single threaded handling of that traffic, and it can too much for one core, especially if you get a lower power cpu. It might be possible to convince the network card to look a bit farther into the packet to hash packets to different queues, but at least it doesn't happen out of the box.

Somewhere around HN there is a guy that did just this for the 25G internet to his home, full talk with slide deck and all. I think he wrote some custom Go code to simplify running it as well.

In general though unless you need >10G you'll come behind on (good) COTS offerings in price and performance. Particularly if you need features like NAT or firewalling where software starts adding latency or performance cliffs at certain intervals while things like a low end Fortigate have high levels of hardware offload.