I would strongly disagree. In terms of setting up OPNSense (I use pfSense, but same concept), it’s easier to just do a PCI passthrough. The alternative is to create a virtual network adapter on your hypervisor, bridge it to a physical NIC, and bind the virtual adapter to the VM. The only advantage to be gained from that is being able to switch between physical NICs without reconfiguring the OPNSense installation. For someone with a homelab, when would you ever need to do that?
My Proxmox server uses a 10Gb PCIe adapter for its primary network interface. The onboard NICs are all passed through to pfSense; I’ve never had any need to change that, and it’s been that way for years.
I don’t mean this to sound overly critical, and I’m happy to be proven wrong. I just don’t see a “plethora of reasons” why doing PCI passthrough on a NIC is a bad idea.