In London? They're usually quite accurate - used to be tracked by roadside beacons so had less resolution but now it's all GPS I think.

Of course sudden traffic, letting an elderly passenger get etc will mean that the countdown won't be the same to the second. But that's going to be the same if you used roadside cameras too.

Generally it should be seen as more of an estimate based on how long the last bus took to arrive from the same point rather than an exact prediction but it's usually useably accurate in my experience.