Originally Posted by wayfun
... These may work for low voltage applications but I think they are poor for 120V.
I agree. As has been indicated already they are intended for tapping/connecting low voltage systems.
They can slice through the low voltage insulation and make contact well, but may not correctly cut through heavier insulation like THHN/THWN or similar material. That insulation can force open and distort the connector causing it to not bite properly. Another issue with this situation is that I would never use them on #14 or #12 AWG solid wire like Romex building wire, just stranded conductors. It is a poor choice for heavier solid wire.
Many people don't like this type of wire connector and will say not to use them at all. As with any wiring method, I believe that it has a place on low voltage work for many applications when in a controlled environment and properly installed (not hanging out exposed under a vehicle for example).
Whoever used them to connect your water heater has demonstrated that they aren't up to that task.