The Expodisc transmits 18% of the source light to give the same results as a properly used gray card. Like the gray card, it can not and will not take into account the intent of the photographer. It merely sets everything to Zone V.
For a literal interpretation of the subject, this is fine. For any other interpretation of the subject neither a gray card nor an Expodisc will make the exposure adjustment required.
We have a tool (Zone System) that takes care of all this, but very few photographers care to spend the time to learn it and use it.