The biggest problem about studying grammar at the sentence level alone is that it gives students no practical application of the information they need to learn or how they will apply what they do learn. Students must be given real world application of they skills they learn.
No isolated study of the grammar of a language leads to any meaningful language acquisition. In his novel, Black Boy, Richard Wright recalls his hunger for knowledge along with his hunger for the ability to express himself well. So, Richard studied and studied grammar books, but they failed to interest him. It was only when he began to read voraciously that Richard Wright also began to write. For, the learner must perceive grammar in action on the page of the novel, on the page of the biography, on the page of the treatise, and on the page of the narrative for language to come to life and to become meaningful.
That is not to say that grammar study is an exercise in futility. After having read something that relates to the individual, then a person can examine those passages and define the structure of them. Also, the student can study structures met in the passages read.