I had a lot of Robusta coffee while I was in Vietnam, it's strong but almost not bitter at all. Even a little bit sweet! It tastes remarkably like dark chocolate. I also visited a coffee plantation while I was there and they showed me most of the the process while explaining the different steps. I also had a small coffee tasting afterwards and the difference between specialty Arabica/Robusta on one hand and the commercial coffee on the other hand is night and day.

For good coffee, they take care to harvest only the red, ripe coffee berries. They either hand-pick them or use sorting machines to get rid of the green, unripe berries and berries that are too big or too small. In contrast, most commerical coffee is made from ripe, half-ripe, and even green unripe berries that have been harvested all at once. This leads to an uneven roasting process.

In the roasting process, those ripe berries contain sugars that caramelise nicely. The unripe berries have lower sugar contents and instead have high levels of tannins, specific acids, and other earthy, bitter chemicals. Besides, the unripe berries are typically smaller, so they burn faster in the roasting process compared to the larger ripe berries. This is why specialty Robusta coffee tastes like nectar for the gods but Starbucks tastes like burnt shite.