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.
If the case is criminal and there are other witnesses not insisting on a niqab, the prosecution may still be able to make a case based on the other’s testimony. If the niqab witness is the only witness or is critical to the case, the case may fall apart right there. It may never get to jury deliberations.
Another thought: if women can claim the wearing of niqabs in court due to religious beliefs, shouldn’t men be able to do the same? Imagine if the defendant, both lawyers and the judge all wore niqabs. And don’t forget the jury, either.
That is my take. And even in civil cases, you claim might well be dismissed. If you testify for the side dragging someoen into court, plaintif or prosecution, the defense gets to demand to see your face.