Define success rate?
So far in ~30 batches I have had two issue batches. One was due to using a fermenting bucket that was Dad's old one and apparently the reason he quit was due to an infection issue( thanks for telling me this after I brew my first batch). Second was a fermenting WAY too hot issue. Both were drinkable, just not great.
Or do you mean tastes amazing vs not amazing. That is like cooking, sometimes you make something and it is amazing, next time you do everything the same and it is meh.