They're illegal for road use in a lot of states, yes, but not private use. So in most states, if you need something for around your property, you're still allowed to buy one. Some states will let you register them for road use though.

The bigger issues are 1. To be imported, they have to be at least 25 years old, so the current ones are from the late 90s. Thus, they have the tech to go with it, limiting their speed.

And 2. They're built and designed for Japanese roads and regulations, not American ones. Speed limits are different there, and as you said, they're better for city use, I'd say non-highway use.

They're legal in my state, and I want one when I can afford one, but I'm also less than a mile from a major home improvement store, and the other two stores I would need to visit are within 20 minutes driving by backroads. But I'm a fringe case, but I'd say for most people who live reasonably close to a Lowes or whatever and are only going to use it for weekend projects would be perfect candidates for a kei truck.

Beyond that, yeah, they're limited :/