The problem is not so much what unbound can handle, that seems to be limited by RAM, the list updates from pfBlocker can be done seamlessly.
The problem you’ll have is that if you have DHCP allocating names to IPs then every time you do so it restarts unbound which reloads everything and that takes time with a big list. My system (N3150, 8gb + SSD) starts getting grumpy after ~600,000 domains but it’s just reload time.
I looked at looking at how DHCP sets up the names to use the seamless method that pfB does, though it may be possible to double up on DNS servers somehow with a clever config.