During testing D4638: Camera Background Images we came across the issues that holdouts and DoF rendering were having render artifacts might be solveable. Investigating into it lead to that the alpha values were all set to 1, so we couldn't do the right compositing of the background images afterwards.
Holdout in combination with transparent film doesn't seem to work. When scene as Film Transparent enabled and I create a viewlayer that is set to holdout, the Cycles viewport rendering shows the transparent area created by the holdout as grey world background, but not the Background images. You can try this file:
Just hit shift+z to go to rendered viewport to see the issue.
It does work fine in 2.79 (using mask layer in renderlayers)