The Minnespota Department of Natural Resources put out a booklet some years back titled 'Woodworking for Wildlife' I use this a reference for my WD boxes, bird houses etc.
I'm about a block from the river and get wood ducks and sometimes owls.
I have a bunch of them in trees and when I set any more out they will be on metal posts in the water. Tree mounted houses are just coon feeders and squirrel shelters. One spring I found two dead squirrels in a nest box, no clue what killed them. The biggest surprize though was the little owl who I woke up when checking the box.
Agreed, If they are on a tree or near one they are not as effective. Mine are all on the water and have been producing each year. Just make sure the hole is at least 5 ft off the water.