I tried Rhododendron viscosum before I fully got my head around azalea and blueberries. R. viscosum does not tolerate my municipal water in combination with a Turface and pumice potting media. It crapped out by the end of my second summer. Probably to alkaline a reaction from the combination of my medium hard water and the Turface & pumice potting mix. If I were to do it again I would use either 100% kanuma, or my blueberry mix, fir bark & sifted Canadian peat, with no more than 50% pumice or other mineral component. Zero Turface. The fir bark and peat blend becomes quite acidic as it ages and will compensate for my 225 ppm as calcium carbonate municipal water.
The leaves of R viscosum are about 2 inches, and don't reduce a lot. That is likely why Arthur Joura substituted R. kiusianum for R. viscosum in the planting, better sense of scale. Similar issue with Kalmia, the leaves are small enough for 2 to 4 foot tall bonsai, but when trying to work it into a forest planting if it is to represent an understory tree, the leaves need to be smaller.
So the substitutions are really for sense of proportions. If one were to actually use sugar maple and hemlock, the forest planting might have to be 4 to 6 feet tall, because leaf petioles and leaf reduction of sugar maple is simply not very good. Leaf petioles pretty much stay the same length. WIth Acer rubrum you end up with small leaves at the ends of long, normal size leaf petioles. It looks weird, like flags stuck out all over. Got to "go big" to get past these problems.