This ARSS thing has two main problems.
First, it's not stealthy for a nut. No matter if it shoots someone down, it cannot shoot everybody, and two snipers can get rid of it.
Secondly, it's a machine, and for the moment, they're more expensive than a meatbag with a sniper rifle or rocket launcher. So numbers will still win the day, and the robot won't be able to know where all snipers are.
The main advantage it has is ballistics, as it hovers above its target, gravity will help, increase range and damage. Snipers on the ground will fight against gravity, which reduces ranges and damage.