There's a tremendous amount of equipment out there that is perfectly safe, 'state of the art,' and not listed. Not by UL, or anyone else.
UL has a strong presence in some industries, and a very weak presence in others. Even control panels are not usually listed; and even a 'listed' panel can be designed in a manner that operates equipment in an unsafe manner.
If the electrician doesn't feel comfortable, that's his call to make. If the customer encounters this with a few electricians, there might be something else going on.
