What are the merits and demerits of essay tests?

1) merits of an essay tests is that if you have a good constructive grammar you get a higher grade but the demerits of an essay test is that if you don't construct well your answer then you get a lower grade.

2) another merit is that if you have studied well then of course it would be easier to answer but if you haven't studied well then you don't know the answer.