By 100%, I mean that it should fail when it does not know the answer, not that it needs to be able to answer 100% of provided questions. It should not just output any answer.
Alternatively, for the problems it knows how to do, a CAS should be as close to 100% as can be reached. But you are right, even Mathematica is not always correct so it is useful to be able to query a solver for its steps.
Alternatively, for the problems it knows how to do, a CAS should be as close to 100% as can be reached. But you are right, even Mathematica is not always correct so it is useful to be able to query a solver for its steps.