My gut is that computing key (given reliable pitch data) is a lot easier than computing polyphonic pitch data (given audio).
For relatively "conventional" music, there are very strong signals of key like beginning and ending chords, and overall note distributions which will generally cluster around one particular scale. For less conventional music, this will be more ambiguous, but it would have been more ambiguous for a human listener too.
For relatively "conventional" music, there are very strong signals of key like beginning and ending chords, and overall note distributions which will generally cluster around one particular scale. For less conventional music, this will be more ambiguous, but it would have been more ambiguous for a human listener too.