It's not all that uncommon for submersible equipment like powerheads and heaters to lack UL listing. I don't think the Koralia powerheads are even UL listed.
In fact it happens so often in this industry that when a piece of equipment is UL listed, manufactuers advertise it like a "product feature" whereas if you bought something ordinary like an electric fan, table lamp, etc, it would be just taken for granted that it's listed.
By the way, you can't accurately measure stray voltage with a multimeter, so I'd say you are safe if you have a GFI and a grounding probe. If you have just a GFI, you could have a problem that goes unnoticed until until something grounded touches the water. But if you have a grounding probe as well, and you regularly test your GFI, you should be able to rest pretty easy.
I can't imagine though that these powerheads would be any more risky than anything else. Most of this stuff is made in China or someplace else like that anyway, regardless of who's name is on the label.