Buses are terrible because they get stuck in the traffic they're competing with. If the bus is the same speed as the cars, people are more likely to take a car.
This can be fixed - dedicated bus lanes mitigate this nicely - but doing so is politically unpopular. And frankly, if you have the political will then you should just build trains. They're more reliable, cheaper in the long run, and have higher capacity.
The main advantage of buses is that they're a great stopgap, and are good for niche/dynamic routes.
Los Angeles has been doing this and the BRT lines are a great way to get started since they’re so much cheaper to build. The last time I was visiting family they looked busy and outpaced the car traffic handily.
One game-changing technology is the way cameras are cheap now. You can put them on every bus and change drivers’ calculation of the risks of suffering consequences from blocking bus lanes or stops from “less likely than being struck by lightning” to “every time”. That requires political will but the technology makes the cost not only low but self-funding.
This can be fixed - dedicated bus lanes mitigate this nicely - but doing so is politically unpopular. And frankly, if you have the political will then you should just build trains. They're more reliable, cheaper in the long run, and have higher capacity.
The main advantage of buses is that they're a great stopgap, and are good for niche/dynamic routes.