Translation:Their dumplings are terrible!
It is ridiculous to expect us to guess what negative adjective to use here. Why does it have to be "terrible" as opposed to "bad" or "awful" or "lousy" or any of the endless synonyms for "terrible". At the very least, "bad-tasting" should be accepted as a literal translation.
Well, I wrote "Their dumplings are inedible" and it was not accepted.
I have to admit that "inedible" was the first word that came to my mind when I saw "很难吃" and that I should have spent a few more seconds to find a more familiar word. Anyway: "terrible" is indeed not the only possible translation.