Leviton Reviews 

Cannot insert twisted stranded wires in a 10 gauge cable into a Leviton NEMA 6-15P plug (5666-BX) holes without strands hanging up and being pushed back although the plug is supposed to be able to be used on a 10 gauge cable. This is very poor and is obviously a flawed design. On another type of plug made by Leviton, I had the same problem with a 10 gauge cable which was supposed to be compatible and brought it to the company's attention and was blown off.

have some brandrex (leviton) rj45 keystones, in their keystone patch panel (which is near impossible to remove the keystones from) nearly every single one has poor contact issues after a few insertions, have to go into the port with fine twezers to pull the pins back up. shocking quality, had better with the cheap china keystones.