Bus stops here: Shanghai lets riders design their own routes

How does that work out for someone in the unpopular destination who wants to leave?

There is a loosely defined route that still needs to be followed. You just shout you want to leave when you are near your destination. Or the driver would ask/shout is there anyone going to XXX area when it is near, you are supposed to say yes otherwise it gets skipped

I guess I’ll add an example. Let’s say the minibus mainly goes from A to B, but pass through C in the middle. Dropping people off at C is often a non-trivial task that may takes a couple of extra minutes so you need to tell the driver in advance

No, I mean, what if there is someone at C that wants to catch a bus, but all the buses are skipping C because no-one already on the bus wanted to go there?

Well, you take other transports. Or call the minibus company and sometimes they’ll arrange for you. Hong Kong is a bit unique though, that most people go to one or two areas for work, so the minibus is probably already full at C in the morning anyway