One of the main issues with TCAS, for me, is that no one knows what the value of X is.
Indeed , and that debate is hindering the obvious solution to our problems : making the RA a command, i.e. an obligation to follow in all circumstances.
In the US, at its conception ,the MITRE/FAA Lawyers prevented this and insisted we call the TCAS avoidance resolutions ' Advisories " and not " Commands" therefore it is called an RA.
But besides this " value of X " technical issue , there are other aspects as well.
I always refer to the " German glider " one , following a presentation by a famous German 747 Capt in ICAO : to explain why he will never agree that following RA a should be re mandatory in all cases he said :
,I am in IFR in class G airspace and I get a climb RA , but just above me are gliders without transponders , my first duty as Captain is to maintain the safe operation of my aircraft and stay below the gliders , therefore I will elect not to follow the RA in this case.
And he is right. So an advisory it remains.
TCAS is designed to work according manufacturer manual, it is stipulated here that pilots have to follow orders to escape, so why argue with that and prefer to follow ATC orders?
Indeed why argue ? Mainly for 2 reasons : one is human : people react differently to automation that others and culture also plays a role. PPRuNe is full of discussions on this issue . Studies shows that even as we speak today 30 to 40% of the " Adjust vertical speed " RAs are not followed by pilots. ( it used to be 60% 3 years ago ) . So it is not only ATC " interference".
The other reason is the risk , the " value of X" mentioned above. If it would have been all without risks, , coupling the RAs to the auto-pilots would have resolved the issue and would have been done.