I like strong coffee and drink it black...it must be that on the rare occasion I have entered a Starbucks, it was not a busy time, because the coffee tasted like it had been burning for hours. I think they just don’t ever throw the coffee out when the crowd dies down. But it definitely is ‘burnt’ sometimes. Reminds me of the stuff we had in the army.
Most of my experience with Starbucks has been via grinding their beans and brewing coffee. I have never found it to be bitter or burnt. I would think that any coffee that sits for a while on a warming plate will taste burnt. One reason I switched to K-cups is that every cup is fresh.
I did work at a Starbucks during Grad School (for the health insurance) The reason it tastes burnt is because the primarily use Asian and/or African coffee beans which typically have high acidity rates. Central American beans are much milder but cost a little more. My favorites are Casa Cielo and Guatemala Antigua. I never drink coffee black.