Am (especially) and Em happen to be very easy piano keys, too.
What I meant is that playing a C major scale is easy, but it gets weird as you work through different keys. There's no consistency, so transposing is non-trivial. Compare that to a guitar and barre chords and capos.
C major is only easy conceptually, technically other scales are easier, such as B major.
In guitar transposition is easy conceptually, but not so much technically, not being able to rely on open strings as much is a big limiter to novice guitarists. And guitar being such a popular instrument, there's a lot of those.
On average technical limitations will dictate keys more than conceptual limitations.
all white keys is easy to play a scale without thinking, but I find it much harder to play well than keys that have some sharps or flats. that pattern of 2-space-3-space on the black key shelps to anchor your hands. I'd take e-flat major--its far easier to play than c-major.
Transposing is harder, but the vast majority of piano litature is never played tranposed--you play the notes on the sheet music. I've tried some of those new layout keyboards like the Linnstrument, and I'd never try to play a Beethoven sonata on one.
To be fair, perhaps most sonatas include transposition within them, only it's written out.
This actually presents a hurdle for me, because if I learned a theme in G, and now I have to play it in D I feel like I know it, so I don't practice it as much as I did initially and it ends up in this weird limbo where I know what I'm supposed to play, but haven't properly trained my fingers.
But there's plenty of stories of Liszt or Chopin transposing music on the spot. Jazz too.
