Well said, @MandoWookie.
The Johnson and Reising were interim substitutes given the lack of availability of the Garand and Thompson, along with production delays of the M2 Hyde. Not "deliberate choices".
As noted the Reising was designed as a police firearm, and it was actually quite a good gun within that context; detritus under the charging handle and individually fitted, non-interchangeable parts simply weren't issues in such a use. The Johnson wasn't that terrible, either. It was actually quite good compared to other semi-automatic rifles from around the world in 1941. There were lots of good designs that didn't go past prototype or limited production simply for reasons of wartime efficiency, not because they were bad. One of the rarest American SMGs during WWII was also one of the best, the Marlin UD-42.
"Piece of junk" from WWII makes me think of something like the S&W Light Rifle. Certainly not either the Reising or Johnson.