Drupal 6 is fairly stable, though if they discover some really critical bugs in it, they'll release a new version of it.

The problem you're running into isn't Drupal, but all the extra modules people write to make Drupal better. Like Ubercart. A lot of them are taking a long time to convert the code to Drupal 6 because they are also taking the opportunity to give their modules a major overhaul to the way they work.