They are not quite as strong as we have been playing them in terms of recursion. The only models that go into the reserve pool during the game are those that are forced to Fall Back. This simulates them disappearing into their tunnels etc. If you shoot them off the board they are dead (and deserve to be!).

The tunneller transports must come to the surface to let troops out and therefore it would be possible to shoot them off the board potentially.

There is no limit to how many troops in the reserve pool can be brought back in a turn.

Makes sense now I have had more time to review the rules. The 3 reserves per turn is overruled by the swarm rule which stipulates that you can deploy any number from reserves.

The nuance of reserves being units that have fallen back starts to make more sense, massive models never fall back so can never be recycled back

We need to see if artillery can deal with the pesky nid transports !