Brass players are more at home in flat keys because their instruments are built that way: with fundamentals of F, Bb and Eb. (Yes, I know: trumpets in A, C and D, trombones in G and probably others also exist!) Guitars have a slight bias towards the sharp side. (Yes, I know: capos!)
On a piano any key is fine, but I can't think of a piece in Cb. Can anyone? Plenty of pieces visit Cb (Bolcom's Graceful Ghost springs to mind) but they don't often start and end in it. So it's rather an unfamiliar key. I could play scales of Cb faster thinking of them as B!
If it was at all interesting a piece of music in seven flats would soon need double flats, and along with the double-flat signs there would be the signs to cancel the double and return to single-flat. Big chords with mixtures of those accidentals are a pain to sight-read: the bars get cluttered, like a street full of speed limits and No Right Turns.
It's odd that Cb major is such a warm, squashy key when B major is such a glittery one.