The justice system mostly works (relative to what its designed to do and the system it evolved out of, which you might both disagree with, but let's not complicate this). By the nature of what it handles (humans) and what you are (human) it will look abhorrently cruel when it fails. And, as with any system, it will definitely continue to fail as long as it is in place.
Assessing an acceptable failure rate is kinda hard – but realistically there has to be one or you will just have to do without any system at all.
So, more interestingly, the thing to look out for is whether a) there are institutions that report on system failures and b) we learn from those failures and correct them at all. As far as I can tell that generally happens in democratic countries.
The next thing to look at is the speed in which we do those corrections. Could they be quicker? Sure. However, it seems like democracies are a tad slow about everything to people everywhere. Or, put differently, we are all wired a little bit too impatiently for how our democracies are currently designed.
I feel that's good thing, constantly scrutinising our systems, keeping them on their toes and improving as we go.