Re: Unlikely to be useful for individual PCs (desktop or laptop)
For servers with high network traffic smartNICs may be useful.
At work we have perhaps 40-50 VM's per server, and all are microsegmented into their own virtual LANs, with only the minimum ports opened between them as needed. Many servers talk to each other via network and all network traffic is routed through external firewalls.
If you had a firewall inside the virtualisation server, you could cut all that extra north-south traffic, and thus save money in the network gear. Network wouldn't be as congested and latencies could be lowered.
You can already install software routers/firewalls and run them as VM's, and route all VM traffic through them. Depending on your requirements, something simple such as pfSense could work. If you needed content scanning/packet inspection (L7), a virtual firewall could really eat into the processing capacity of the VM host itself, which would be bad. In VMware load balancing scenarion where DRS could move any VM to any host in a cluster, you would need the software firewall in every host, and the configuration would need to be kept in sync as well. If it is a commercial solution - expect to pay for each host and then some.
If these SmartNICs can do this without any meaningful performance penalties, and their management isn't a total PITA, they could be quite useful. They will likely cost a lot of money, but so do other solutions as well.