To me at least, the sound of simultaneous, precisely integral multiples of notes loses rhythmic point beyond a certain complexity and speed, the aural result being the same as completely asynchronous runs being played. Two or more truly asynchronous rhythms, i.e. not just note strings, going on simultaneously, on the other hand, can be very exciting, and easier than might be supposed, in improvisation anyway, provided you don't actually think too much about what is going on.