The Sheffield ones are rather hit and miss: sometimes they show the number of minutes until a bus is expected to arrive at a stop, and sometimes they just show the scheduled time of arrival. In the latter case the bus often simply fails to turn up, although they do sometimes show if a bus that is due has been cancelled in which case they flash between the word CANCELLED and the destination of that bus every few seconds. I would guess that the number of minutes until arrival is only shown if the bus has GPS equipment that is in working order.
Yes, with most systems, the predicted time (as in '5 mins') is only shown if the system and the bus are talking to each other. Most systems default to showing the scheduled time (as in 15.10) if the system can't see the bus.
That may be because the bus is temporarily in a dead spot, or the kit on the bus isn't working, or the driver hasn't logged in properly (although since it's now usually part of the ticket machine login that's getting rarer, although a bus can 'disappear' during a driver change-over), or the bus simply isn't running.
The alternative approach (and I believe TFL's countdown works on this basis) is that if the system can't 'see' the bus, it doesn't show a prediction. This means predictions go off during the driver change, and, while GPS dead spots are rarer in London, I did encounter a situation where one bit of Lewisham had become a dead spot (think there was some equipment defective in the area) which meant that buses fell off the system for a few minutes then re-appeared. Some people would assume that meant a bus had been cancelled and start walking, only to be passed by the bus a few minutes later.
No system is ever going to be able to deal automatically with a bus that's been running normally then breaks down, or gets stopped due to a sudden traffic incident, and system will assume that if bus is on stand at the terminus, then it's going to leave on time, which may not happen for a number of reasons - some may be known in advance (if it's known a driver duty is not covered that day), some may not (if driver presses 'engine start' and nothing happens.)
Yes, most systems now do have the facility for someone to make a manual intervention for cancellations, or to put warning messages up at particular stops, or (in many cases) to suspend predictions being shown at a stop that's closed, or where buses are diverted away from normal line of route. But that requires there to be a 'someone' to do it.
All these things require management will and staff resources at combination of bus operator and local transport authority. Both financial constraints and political / management thinking often means that getting shiny new kit installed is seen as a priority, having staff getting the data right in the first place, and at any service or stop changes, and dealing with day-to-day issues is often seen as not a priority, and managers talk about 'looking at the big picture' when what they mean is they can't be bothered to deal with what to them is minor detail, but to the passenger waiting in the rain, is about the journey they are making (or failing to make because that bus isn't running.)
With some systems, only the local authority people have got the access to put messages up, which is all very well if the incident happens during office hours, or the local authority has out-of-hours staff who can deal with it, and if bus operator tells local authority.
All of these will apply just the same whether the real time information is shown on a physical piece of kit in the bus shelter or on a smart phone app which some people won't have, others won't have a smart phone, and others won't have credit for the data.
