Agreed, but as post 13 points out, it does not prevent the defendant from confronting the witness. As for the jurors, if someone testified wearing a niqab, it would be quite reasonable to ignore their testimony. Who is really under that niqab? Why are they saying what they are saying? What is their motivation for testifying in the way they are? Their testimony might be rendered worthless by their insistence on covering their face.
Not to place to fine a point on this. But if a criminal defendant cannot expose the witness’ demeanor as an integral part of cross-examination, then the witness’ entire testimony may have to be dismissed and, since this is a constitutional right (unlike in civil cases), the charges themselves would have to be dropped.