It works surprisingly well in practice. The key thing to remember is that you rarely connect identical devices together.

A laptop and a power bank both support both modes, but the laptop will have a "prefer sink" policy and the power bank will have a "prefer source" policy. As long as you don't connect two laptops or two power banks, it'll work out just fine.

Moreover, it has an override mechanism in case you do connect two identical devices. If you do connect two laptops together for data transferring, the OS should be able to let the user override the power flow direction - or even disable charging altogether.