After a few weeks, if you don't address your postural issues, you'll discover that you can have terrible posture on a yoga ball just as easily.

Absolutely, but it's a great way to strengthen the core and work on posture. It's always going to take work of course.

This puts a lot of pressure on the lower back. Sitting down for long periods without any back support doesn't seem like a great idea to me.

I would argue that half an hour to an hour is no one's idea of long periods.

You said starting so I assumed there would be a progression. Overall it doesn't seem like a good idea to me because it puts the whole weight over the lower back without any back and arm support (like the ones that exist on a good office chair, they're recommended for a reason). Also, there are a ton of good exercises for the core that don't stress the back, like planks for example.

Anyway, I recommend that people ask their doctor for advice or at least Google up a bit about the pros/cons of this and try to find articles from reputable sources. I did that some time ago and wasn't very impressed with the results.