The trouble with TCAS is it's only as good as those using/believing it.
The DHL/Tu154 accident some years ago is the perfect Eg of that. The pilots would have been shocked to see that they where in conflict with each other out there at night in almost vacant skies over Europe but there was other mitigating factors involved (equip off line for maint etc. Awful story that one, very sad:-(
It does show that TCAS although is a great tool can just be another tool that makes the pilots 'toolbox' heavier.
Windows are great if yr visual! TCAS doesn't know if yr vis or not.
Wmk2