Firstly, condolences to all those affected by this tragic accident.
Secondly. Far, far, far too early to say whether anyone was (or was not!) at fault!
Now
TCAS works great inside Controlled Airspace.
TCAS works great when everything you could hit, has a transponder. Preferably with an altitude read out.
TCAS works great when pilots have been taught how to use it, and follow it’s commands religiously. Whether or not they can actually ‘see’ what they presume to be their conflicting traffic!
From what I’ve heard of the area around WAP (and other popular GA locations, the danger is, pilots would be swamped by information, and deconfliction manoeuvre demands.
TCAS does not work great when pilots start to ignore it due to it sucking up too much of their attention. Much of it inside the aircraft, when they should be looking out.
TCAS can be confusing, and frankly rather scary when it commands conflicting manoeuvres due to multiple simoultaneous threats. Or if the other aircraft does not follow coordinated actions.
Coordinated actions often require specific rates of climb or descent. In such situations, TCAS requires Instrument Flying skills, the trajectory of which may also take you into cloud.
Knowledge of TCAS seems a little ‘rudimentary’ by some on this thread?
TCAS is not a panacea. It simply brings different problems. Some of which, an ‘average’ GA pilot might struggle with?