I wouldn't call it bitter with all the sugar they jam in. So few end up drinking black coffee from them.

I would only drink Starbucks as a dessert drink at this point.

I generally only like cold brew coffee (not as bitter, and I prefer cold drinks) and their unsweetened coffee is, as you say, a very dark and bitter roast made more bitter by brewing.

There are better dessert drinks out there, and making a milkshake at home is easy as jamming some shit into a blender.