Containment vessels for hundreds of deadly diseases
It would appear that there's a theme of forcefield containment going on with things that are difficult to replicate. We also learn that photon torpedo warheads are easier to steal from a Starfleet facility than to replicate at home...
Possibly the difficulty of replicating intricate forcefield machinery is what's keeping Commander Maddox from producing a dozen Datas to vivisect, or Starfleet from creating starships at the push of a button? It might be something they can do (since there's nothing the transporter explicitly couldn't do), but won't, because of the complexity and thus the time or other expense required?
The difficult replicating of explosives wouldn't quite fit that picture. But in "Night Terrors", the power shortage aboard the ship was particularly severe. Or at least the loss of propulsion was quite total, even though nothing indicated that the anomaly of the week was actually grabbing the heroes. It simply seemed as if they had lost the power to fire up their engines, even the feeblest ones. Which is a bit inconsistent with them being able to consider a deflector dish burst as a means of escape... But apparently the issue is the rift eating up all output power, even if there's lots of energy in the reserves. If it eats things as diverse as propulsive power and deflector output, then it could eat replication energies, too, and we'd learn nothing much about the normal capabilities and limitations of the replicator.