I asked exactly the same question a few years ago.
Since then, I have used it many times within the EU. I like that I can easily carry it in my wallet and hence I can pop over to another country any time that I feel like it (well, I could prior to Covid). A few countries beyond the EU and EEA will accept it but primarily it is an EU document. I would check carefully before relying on it to enter a non-EU/EEA country as I expect that the rules could change. A downside is that it does not work in the automated passport gates or many self-check-in terminals.
One use that I have not attempted yet is entering the UK after Brexit. I remember in the days before computers (I am old) that, when I entered the UK with an Irish passport, I would be looked up in a large book (The A to Z of terrorists, I expect). The only exception was when I entered just after my sister. I guess that there was no entry for our surname and he did not have to check me.
Once, in Amsterdam I was rushing for a connection and I joined a priority queue. My need to use the queue was questioned. I explained my short connection time. This was accepted but the official said: why not use the automated gates as there is no queue. I explained that I was using a passport card. He was surprised, he did not know that Irish passport cards existed.
For use within an EU country, e.g. checking into a hotel or renting a car, it hs always been immediately accepted without any query.