handling and steering difference for stem/bar reach combinations?
The exact difference can't be easily quantified in terms of how short is too short. The closer in you get, the more amplified an effect your steering inputs will have, creating a feeling of touchiness. It's not optimal, but it's a pretty subtle downside and well worth it if it's what it takes to get comfortable.

Comparing bars with different amounts of reach (and drop, especially since most bars with smaller reach numbers will also be shallow drop), a major consideration is that the smaller the number, the less variance is created between the different hand/body positions. That's why it makes sense for the size of the reach/drop numbers to scale with the size of the person.