The difficulty of porting NIC drivers is probably not in differences in the userland API; kernel drivers don't likely pledge anything. But OpenBSD and FreeBSD diverged a long time ago, and I'd be surprised if their kernel APIs are very close anymore. How to detect and interface with devices is probably a bit different, and rx/tx packets will be different too.

I think most of the vendor supplied NIC drivers in FreeBSD are BSD licensed, so that shouldn't be an issue. I checked Intel, Melanox (now NVidia), Cavium/QLogic/Broadcom, Solarflare. The realtek driver in the tree is BSD licensed but not vendor provided; the vendor driver in ports is also BSD licensed. I'm not sure if there's a datacenter ethernet provider with in kernel drivers I missed; but I don't think license is a problem here either --- anyway you could ship a driver module out of tree if it was.