TDL suffer from things that blade spawners and even zombards suffer from.
A certain critical mass, based on a relationship of your ships to enemy ships, occur.
Below critical mass #1, they completely dominate.
Above a larger critical mass #2, they drop off in effective ability drastically.
Between these two masses you have a mix.
This is true for any ship class, where your own forces dominate the enemy so much for whatever reason you suffer few losses, but it is really visible with things like tdl, spire blades, zombards, etc.