The hidden tesla can be used effectively when you put it behind buildings that has higher hit points such as the storages or even the town hall.
A hidden Tesla remains hidden until enemy troops come close enough.
They add a surprise factor to enemies raiding your base.
When upgraded, they can cause huge damage on enemy troop.
Hidden Tesla can also attack both air and ground units.
The favorite target of the hidden tesla is the PEKKA, but it will show itself with any troops that comes near them.
Proper placement of your hidden tesla on your base will ultimately determine its effectivity.