What are the present problems of TCAS and ACAS up-to-date?

and what are the possible improvements and developments for these devices?
thank you
Update: it may not be allowed here but please give me a space to reall thank you for you very detailed answer and i really would like which once is the most widely used in the world TCAS or ACAS as i asume its just a name deferential but i would like to know the core deferences between the two systems as well
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