Because holdouts workflow involves multiple AOVs, rendering with adapt all enabled can reduce noise at the cost of slower speed as it may trigger more samples based on noisier AOVs.
By default, light sources are considered holdouts and do not illuminate other holdout objects directly. This prevents double contribution between lights used to illuminate non-holdout geometry and the illumination already captured on the plate image.
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When compositing the image if the shadow matte is part of the beauty, it is a simple over operation. This will provide you with the correct shadow density and look based on the render settings.
However, this means adjusting the shadow density is tied to your beauty alpha, you will need a mask to prevent shadow adjustments from ruining your CG object composite. Cryptomatte may handle this for you as part of your usual arsenal.
If you output the shadow matte separately then you can adjust the shadow density separately, however you may have a coverage issue, the solution is to merge your alphas together in comp from the beauty and the shadow matte. An example using Nuke is here.