[sgxin]

But HK public buses got display right?

I've never taken their buses, but after searching online, you are right.
Their bus readers do display the fare and card balance.



And according to this vlogger, the HK Octopus card can even be used in the China mainland when riding the public buses!
https://www.ixigua.com/7211454113148731942

I took the Airport train to HK main island last year. I vaguely remember seeing the fare.
Like the case below. So this means the HK card system supports reader/gantry display of fares and balances.
Those train station gantries that can't are likely older ones that have yet to be upgraded.


https://www.ixigua.com/7280834302407868980

The Octopus card reader for retail stores also shows the transaction amount and balance (in 0.3sec).
Picture from Octopus website itself:


https://www.octopus.com.hk/en/business/index.html