6 have been built (plus Powder Keg), Only Ring Racer in Germany is the only one pulled down, but it only operated for 4 days and was closed because it was not economically viable (built at an F1 race track, not a theme park). Maxx Force is reliable - usually down because of train/sensor/block issues not launch system issues.
The 4 in China are currently SNBO because Bullet Coaster had a roll-back on the top hat and collided with another train in the station. All of the air-coasters at Happy Valley parks stopped operating in China the same day.
Powder Keg is an air-launch retrofit of a Premier Rides Water Coaster, and it's extremely reliable.
I wouldn't buy a coaster from S&S, but the air-launch system is just a horizontal launch tower, which are generally one of the more reliable rides available. If the goal is to keep the existing Intamin ride (and trains) and just change the launch, then S&S would absolutely be an option. Building the compressed air launch systems are the one thing they do really well.