We recently came across with an issue where Magento intentionally disabled discount for Gift Card Type Products. It was there in Magento 1 and I totally understand that discount on gift card products may be leads to any fraud since Gift Card is similar to Cash value. But I noticed that in the code it is checking if it's Gift Card Type product then no cart rule apply. This check needs to be done at Admin Configuration, so if merchant want to enable/disable then should able to change that easily without any code change.
We have Physical/Virtual Gift Cards Setup. We created a Cart rule where we can give 10% discount for above products. While looking into code at following file we noticed that it is invalidating gift card type products that's why we are not able to give any discount.