Userspace isn't magic. (Unfortunately the "userspace networking" people have done a poor job of explaining it.) When you're serving static files like Netflix, you want the storage drivers, filesystem, cache, network stack, and NIC drivers to be in the same address space. Whether that address space is userspace or kernel is not that important.
Kind of important from a security/stability perspective, no?
If there was a userspace I/O stack that's as well-tested as FreeBSD there might be a security advantage, but there isn't.