I just figured this out trying to do something similar based on a time constraint.
When setting your symbology;
* Define your Unique Symbols
* In Transparency click on "Attribute Values" and in the next window select "New Expression"
* Set your Transparency, in your example set the Low Values to 50
* Click the pencil to write your expression
I have included my screenshots as examples. Based on what you wrote I think your expression would be something like
IIf($feature.Reviewed = 'Yes', 0,1)