The major problem with TCAS is that it only works with aircraft with transponders. When I was still a radar controller in class G airspace, I had many instances of TCAS RAs where the pilot action could have resulted in a climb or descent confliction with non transponding traffic. In my opinion it's not safe to place total reliance on TCAS in class G airspace, and any pilot who does is negligently risking the safety of his own aircraft and other aircraft which (in the UK) are not required to carry a transponder.