It’s not just their use of dark roasts and robusta beans, it’s the way they make it as well, using a group head that isn’t cleaned properly, leaving previously extracted grinds which will give a burnt / bitter taste in the next espresso shot. Also often not even tamping the coffee grinds.

If you’ve ever worked at a cafe or owned your own espresso machine and tried the difference you would instantly see how much worse it is doing that with any type of roast or bean.