Hamburg's busses don't have coin boxes or sell tickets/fares? You don't have any mechanism that requires swiping a card, tapping a device, showing a card/ticket to an operator, or otherwise proving you have, in fact, purchased a fare?
It's pretty quick, but on the busiest bus routes it slows things down.
Edit: Reading up, it looks like they don't really have those things, and rely on a combination of very infrequent inspectors (which do slow things and cost money) and the honor system.
In Seattle, we tap our cards on something that looks like this: https://seattletransitblog.com/2011/06/28/dont-tap-orca-here...
It's pretty quick, but on the busiest bus routes it slows things down.
Edit: Reading up, it looks like they don't really have those things, and rely on a combination of very infrequent inspectors (which do slow things and cost money) and the honor system.