Compared with light railway/tram, they need more energy per km traveled due to friction and malleability losses in the tires, they emit fine dust from tires and brakes, and usually they pack significantly less people than a railcar so you need more vehicles and especially more drivers for a bus network.

I'll admit though, a bus network is faster to set up and with mini-buses the size of a MB Sprinter van cheaper to operate even in challenged suburbian hellscapes.