In each story, the gimmick(tm) is thoroughly examined and extrapolated in an internally-consistant way, but that is excellent world-building, and independent of genre.
1. As exemplified by Tower of Babylon which wouldn't be out of place in a fantasy anthology.
2. Understand is probably his number 2 "hardest" SF story. The way it is told is closer to a character study on the effects of human super-intelligence (unbelievably authored in 1991). Exhalation is no. 1, and it's focus is still very "soft" SF.