Humans are fallible. The review system is designed to reduce the number of really bad decisions made by humans, with the assistance of technology and a third party reviewer. Unfortunately, the third party reviewer is also human and fallible. The technology can sometimes clearly assist, other times it does not.
I'm a huge fan of the review system and believe it works (in all the sports I watch). Really bad decisions are being reversed and that is what it is designed for. But I will say, umpires should always first have to make a decision and the third umpire review system should only be used to over-rule a poor decision, whether called on or not. That is, if the reviewer has any doubt, then the original decision should stand. The number of over-ruled decisions should also be a KPI for umpires annual performance reviews and made public. In cricket, pretty much all the worst decisions in history were made without the inclusion of a review system (except for the 1999 howler by the Sri Lankan umpire Pathirana in a Sri Lanka vs Australia one dayer when Gilchrist was wrongly given run out. That decision was so bad, Pathirana was suspended for one game of umpiring.
Pathirana Gilchrist shocker)