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Even though they are getting water pretty regularly, it may not be enough for them to establish and they may be going into transplant shock.

Plants were not meant to move, but we humans like to move them. Some plants, like annuals and small perennials, can establish quickly and you will only see signs of transplant shock in the first few weeks or so. But plants like trees and shrubs establish more slowly and you can see signs of transplant shock for up to 6 months after they are planted.

Lots and lots of water is the key to plants establishing well, so trees and shrubs need more water that first year than they will need later on.

Basically, I would recommend increasing water to the failing plants. Maybe providing a little shade for them too, if you can. This article will have more information on dealing with transplant shock:
https://www.gardeningknowhow.com/plant-problems/environmental/learn-how-to-avoid-and-repair-transplant-shock-in-plants.htm

It is possible that the shrub is being eaten by slugs since they can cause damage like this. They only come out and feed at night, making it much more difficult to see them. However, you should notice a distinct silvery colored "snail trail" throughout the area. This article will also help, should this in fact be the offending party: https://www.gardeningknowhow.com/plant-problems/pests/facts-about-slugs-and-how-to-kill-garden-slugs.htm

Here is an article that you may find helpful: https://www.gardeningknowhow.com/plant-problems/pests/insects/treatment-bagworms.htm

Here is an article or two that you may find helpful: https://www.gardeningknowhow.com/plant-problems/disease/how-to-get-rid-of-sooty-mold.htm
https://www.gardeningknowhow.com/plant-problems/pests/insects/whitefly-control.htm

You can trim viburnum any time of the year without hurting the plant. But pruning now may lessen the number of blooms for next year. If that is ok (it will recover for the year after), then you can prune. If you have not read it, this article will help: https://www.gardeningknowhow.com/ornamental/shrubs/viburnum/viburnum-pruning.htm

You can typically prune it back by a third without hurting the shrub. However, don't take more than that in one given year. If you need to cut back some more, then wait until the following year to do so. This article will help: https://www.gardeningknowhow.com/ornamental/shrubs/viburnum/viburnum-pruning.htm