I can't comment on the bus line example, but the New York Times had a great write-up (https://www.nytimes.com/2017/12/28/nyregion/new-york-subway-...) about subway building in New York. At least in New York there's a lot more going on than the reasons you point out. Particularly damning is the fact that Paris is successfully building subways at a 10th of the cost in far less time despite having even more constraints around digging.
The recent additions to the Rome Metro were also built faster and cheaper compared to, say, the Central Subway in San Francisco, despite all the archaeological artifacts in Rome slowing down the digging.
