TCAS works remarkably well for (lack of a better word) bastardized system. Transponders were never originally designed for an application of this type, and the interaction with this and air data systems for airborne collision avoidance were an afterthought.
There are still some limitations tied to the basic design and antennae…for example, one will never see a lateral escape maneuver due to the lack of accuracy in azimuth (range is very accurate). Down the road, ADS-B may offer lateral escape maneuvers, if used as a collision avoidance device.
Our airline operates with a CDTI that views TCAS targets along with correlated ADS-B/TCAS targets. The TCAS lateral “hop” vs. the rock solid ADS-B targets is very obvious at low ranges.