This is the kind of thread I tend to shy away from, where there is a lot of personal experience and history, and rather less fact.
But like others I have found the true facts hard to come by, and I will contribute my experience as well, as I am interested in other's experience.
I got my first Irish Passport Card almost the day they came out. You can order them directly off your phone and take the photo on your phone. I was living in Ireland at the time. It is separate from the book passport, with a completely different number. You require to have a book passport to get one and its validity is 5 years of the validity of your book passport whichever is shorter.
I used it many, many, many times to travel to Europe (which then included UK). At the very beginning a few people said they could not recognise it including Easyjet and Port Control at Dover, but I stood firm. I never brought my passport with me on these travels - that seems to me a huge advantage. If you lose your passport card, it doesn't invalidate your book passport and vice versa, so at least within Europe you have a 'spare' safely at home.
I was getting the plane back from Brinidisi to UK last autumn, having walked in Italy for a month or so. I had come in on my passport card with no problem. As I was leaving the officer raised a query and told me it was not valid for my return to UK from the EU. Brindisi is a small, friendly, provincial airport and I asked him was he sure. He said he thought he was sure! He asked another officer who told him it was alright. I realised only then that I
myself was not at all sure. When UK was in Europe it clearly was OK. Is it still?
Can you show me where that is written?
I am not in any way doubting what you say, and I think it
should be true.
When I walked the Via Egnatia on from Brindisi to Thessaloniki I was surprised to find, before I went, that I COULD use the passport card for entrance to Albania, and I did, but on that occasion I had to bring my passport as well as North Macedonia does not accept it. The information for Albania is documented
here. And yes I know it says that a passport is 'preferable' but I used the card. I didn't come through Tirana airport though, I came through the port of Dürres.