I have spent a lot of time researching these and IMO they are UL listed for use in 'normal' homes and sometimes concealed.
That in no way means I think it is a good idea.
The NEC addressees these in 2002 334.40
334.40 Boxes and Fittings.
(B) Devices of Insulating Material. Switch, outlet, and tap devices of insulating material shall be permitted to be used without boxes in exposed cable wiring and for rewiring in existing buildings where the cable is concealed and fished. Openings in such devices shall form a close fit around the outer covering of the cable, and the device shall fully enclose the part of the cable from which any part of the covering has been removed. Where connections to conductors are by binding-screw terminals, there shall be available as many terminals as conductors.
They are also addressed in 550.15(K)
(K) Component Interconnections. Fittings and connectors that are intended to be concealed at the time of assembly shall be listed and identified for the interconnection of building components. Such fittings and connectors shall be equal to the wiring method employed in insulation, temperature rise, and fault-current withstanding and shall be capable of enduring the vibration and shock occurring in mobile home transportation.
and 550.19
550.19 Interconnection of Multiple-Section Mobile or Manufactured Home Units.
(A) Wiring Methods. Approved and listed fixed-type wiring methods shall be used to join portions of a circuit that must be electrically joined and are located in adjacent sections after the home is installed on its support foundation. The circuit’s junction shall be accessible for disassembly when the home is prepared for relocation.
As far as approval in the state I live (MA) the AHJ or the inspectors can not refuse to accept UL listed equipment used in compliance with listing and labeling.
