Machine intelligences seem likely to vary in their desirability to humans.

Technically true. However, most naive superintelligence designs will simply kill all humans. You've accomplished quite a lot to even get to a failed utopia, much less deciding whether you want Prime Intellect or Coherent Extrapolated Volition.

It's also unlikely you'll accidentally do something significantly worse than killing all humans, for the same reasons. A superintelligent sadist is just as hard as a utopia.