In case of using expression in standalone.xml then web admin console does not show the evaluated expression and instead of it 0 is shown. In other cases you can see error message of the web admin console saying "Failed to parse response".

As I was testing the behavior of web administration console just on several places I can't talk about behaving of all subsystem.
This was hit on ejb subsystem.

You can try to set following tag like this

<strict-max-pool name="slsb-strict-max-pool" max-pool-size="${some.var:20}" instance-acquisition-timeout="${some.other.var}" instance-acquisition-timeout-unit="MINUTES"/>

In case of max-pool-size you'll see value 0 instead of 20 in the console. And nevertheless that you run the server with -Dsome.other.var=5 you'll see the parsing error because of the instance-acquisition-timeout attribute being set with expression.