For example, in Chicago car ownership has increased as wealthy people move back into the city, and quality of bus service has declined because buses are also stuck in all the congestion that the cars generate. The city has started creating dedicated bus lanes, but they aren't actually improving the situation much because painting the right lane red doesn't actually prevent it from being jammed full of cars waiting to make right turns and double-parked delivery and rideshare vehicles.
Buses are great for roads where there isn't much traffic. However if traffic is bad you have enough demand that a train is better investment.
I am strongly coming to suspect that a lot of the talk about buses being all you need is fueled more by a sort of NIMBYism than by a fully-informed position. Buses are great and good transit systems need more of them than they do trains, but an ideal urban transit system has both.