From the map on
https://www.uber.com/cities/charlotte, on the South edge of the city, it looks like they only service inside the 485 loop. Maybe you could drop your pin inside the boundary and then call the driver to see if they would cross down to Ballantyne?
As far as wait time, if I'm planning ahead, I usually open the app a couple of times at a similar time of day to get an idea of how many cars are in the area.
FWIW, last weekend I took a cab in Charlotte, the fare+tip was $23. For the return, Uber-X was under $9. So it's the way to go, if you can make it work.