I haven't used a foldable. Is it hard to click on buttons, for example, that happened to be place down the seam? Or has that type of thing been long solved?

The seam doesn't really make a difference for clicking. It's really at the point of just being a large phone screen. It's certainly easier than interacting with elements on the edge of the screen.

It is if you hold it half-open like a book -- you can't get your finger into the middle. Apple has added some sort of support for this scenario in iOS that they were demoing for the Duo.