As far as I'm aware, Talent 100's ATAR calculator can generally be considered somewhat accurate. Of course, this may be subject to what exactly you define as accurate, i.e. what is the maximum gap between the predicted ATAR and actual ATAR above which you would classify it as inaccurate? In my case, when I received my HSC results, the ATAR that the Talent 100 ATAR calculator predicted was 0.35 higher than the ATAR that I ended up receiving.
On the other hand, the Matrix ATAR calculator seems to apply a harsher approach in terms of predicting students' ATAR's, while retaining a degree of accuracy. The maximum size of the gap between the predicted ATAR and the actual ATAR above which you would consider an ATAR calculator to be inaccurate also applies here. In my case, when I received my HSC results, the ATAR that the Matrix ATAR calculator predicted was 0.2 lower than the ATAR that I ended up receiving. Based on this, one could infer that a student's actual ATAR may fall between the ATAR predicted by Matrix and the ATAR predicted by Talent 100.
I hope this helps!