It probably gets 95% of the calls right. FIFA says 99.3% but I don’t think so. Still, if it didn’t exist everyone would be crying to implement technology like they did before. Think of all the goals/no goals that have been rightfully overturned. Even if there are some horrible calls sometimes, the good calls outweigh the bad ones.
And a lot of the egregious errors are when it fails to overturn an already incorrect call by the ref.