[subject] + 很 + 难 + [verb]
[subject] + 很 + 难吃 / 难看 / 难听 / 难喝
难 before a verb makes "hard to do". The thing that's hard to deal with is the subject, and the whole phrase behaves like an adjective, so it takes 很, 太 or 非常 in front.
With a verb of the senses, 难 means "unpleasant to" rather than "hard to". These pairs are everyday words, and each is the opposite of the 好 word: 好吃 and 难吃, 好看 and 难看.
难受 is a feeling word of its own, "feel awful", in body or mood.
Read literally, 难看 should be "hard to see", but it means "ugly". 黑板上的字很难看 says the handwriting on the board is ugly. To say you can't make it out, use 看不清楚.
Like any adjective, a 难 phrase takes 很 and not 是.