Thing is that most articles on coffee will say that robusta tastes bitter. And yes, if I think about the cheaper coffee varieties one can get in the supermarket, which are almost always pure robusta, the coffee tastes bitter and astringent.
But with these higher quality Italian espresso beans, I’ve come to appreciate some robusta in the mix as it can result in a very sweet, chocolate-y flavour. And with a good crema! Whereas 100% arabica can be a bit astringent instead. Usually in Italy the coffee you get in the local bars are made with this mix of say 60% arabica and 40% robusta (the ‘bar’ melange’).
Found a nice article on the subject.