Ralph Betza's work suggests doing mobility conditions based on about 60%
of initial piece density. This is 30% for FIDE Chess and 26.4% for Unicorn
Chess.
If Ralph is correct and this is the best value for overall mobility
ratings, the Unicorn is measurablly stronger than the Queen with respect
to mobility, as break even occurs at 22% piece density.
One overlooked Queen advantage that tends to even out the non-mobility
evaluation factors: The Queen has the King Interdiction ability and the
Unicorn lacks it. King Interdiction refers to the ability of a Rook (or
any piece having a Rook's move) to confine the enemy King to a certain
section of the board by attacking the entire length of a rank or file so
that the King cannot cross it.