tl;dw: many of these adapters are so poorly made they might catch on fire. They put the (insulated) wires in and then pour in something kind of like hot-melt glue around them (to hold them in place, and/or for extra insulation, not sure). Sometimes the wires are well separated within that medium, but sometimes they're touching, and possibly the wire's own insulation is melted by the glue. There also seems to be some contaminants in the glue, and possibly corrosion from the way it was soldered.
At the end, he shows a better kind where crimped wires are put into hard plastic channels, which reliably keeps them in place without the same risk of compromising their insulation.
