There are enormous downsides including mechanical reliability and weight.
Raytheon is already selling a system that assists a human operator in drone targeting, then knocks them out with a laser emitter. The whole thing fits on the back of a Polaris off-road vehicle and runs on electricity. That means the ammo is a gallon or two of fuel.
That's why Raytheon developed a laser based anti-drone system. Electricity is cheaper than bullets.