If you consider in small city I live in, Brussels, 1 million inhabitants, we have, I would guess 50 different bus routes (we have a Metro and tram system too). Looking at the real-time maps right now I see some route have 9 buses running, some 5, some, others just one (they are short routes), others 5. Let's assume 250 buses in total.
Now what's the average fuel consumption - quick scout around the Internet would imply around 2km/L for inner-city and I would estimate that each route is averaging 15kms and takes 45 minutes to complete (based on my experience of using). So that would imply in a day 6:00-00:00, 18 hours, with 15 minutes rest, it's 18x15x2=540L of diesel - I assume the bus has a huge tank and can hold this capacity without the need to return to depot. Maybe they can't go all day without fuel, maybe they cycle one out of the route and add one in every hour, I don't know.
But then, what capacity battery would be required to complete a day's work? I'm guessing it would be the same as the semi, which we don't know, but extrapolating from the consumption/km I think it's going to need around 700kWh. Now imagine the infrastructure needed to charge 250 buses in 6 hours, all in parallel. That's quite immense, not only from installing such super/mega-chargers, but on energy-drain. OK, would be distributed around several depots, but it's going to be challenging.
Given the above, I think FCH would be more practical. Not for general public use though, that's a no-go, totally.
Sorry for the long post, so early in the morning...