E-major is inherently out of tune and unstable, especially on the piano.
Excuse me? Due to equal temperament, E Major is just as stable and in tune as any other key,
especially on the piano!
Only true in theory, Jim. How many really good piano tuners do you know?
Do you really find all 5ths on a piano identical?
You answered your own question. If the piano is tuned properly, by a trained professional, then all the notes will be equally "in tune" and all the 5th will be identical. It goes without saying that this doesn't apply if you have a bad piano tuner. ;D