优点好说啊,一般说：You have done an excellent job.
I appreciate your work.
缺点就麻烦一些,要说得委婉：
We know you have tried hard,but there is more you can do in + 应该提高的方面.（例如 efficiency 效率）
There is still some room for you to improve +应该提高的方面(例如,your communication skills交际才能).
If you +可改进的方面,（例如,do some proofreading 校对）,it would be much better.