I think a lot Krautrock of the 1970's was 'difficult' - thinking especially about Can, Faust and Neu! The Tangs started out that way but by the time of Encore etc. they were far more shall we say, tuneful. That reminds me - working from home today so will have to look it out...
Dark Side of the Prog. Allegedly the last Album Kurt Cobain listened to before he ventilated his head... Not a Radio Friendly Unit Shifter, in retrospect I'm amazed Fripp didn't forbid John Wetton to smile on the Cover Photo.
https://www.youtube.com/watch?v=OfR6_V91fG8