I share your frustration on this one. Many times I've checked for elusive ancestors in "updated" record sets, and still found nothing.
One other thing I have discovered with the London baptisms, marriages and burials is not all of the images are indexed; far from it in fact. Recently I have been trawling through the images of St Andrew Holborn, because they are not indexed in the dataset.
I wonder if other record sets are the same.