What is the purpose of prohibiting union coupler in concealed area?
Can't use bushings either (or street elbows, afaik). Presumably because of the existence of exterior threads on a fitting which makes it more subject to corrosion, each thread of which is an inherent shear plane.

"Caution: installing a bushing in a gas piping system in a concealed location can create a potentially explosive situation." – Residential Construction Academy: Plumbing, By Michael A. Joyce, Michael D. Joyce, Google Books