The introduction of DRS is a great way to reduce pressure from umpires as now decisions can go upstairs and chances of error are zero in that case. DRS is not welcomed in start by BCCI but with time they have embraced that change.
Zero? No way. DRS decreased errors for sure, but it cannot resolve every single error just like similar systems in other sports.
IMO, DRS has more than 98% accuracy. And that is the maximum we can expect. When was the last time, we heard someone complaining about any of the DRS decisions? I know about only 2-3 cases, out of thousands of matches. So it is very rare.