Like most of the game, the rapids look really, really awesome from a distance at first sight, but as soon as you start playing with it yourself the cracks start to appear and it turns out to be half-baked.
The way rapid rivers interact with terrain is possibly worse than the paths handle terrain. The buried track generates extremely jaggy edges you can't smooth out without exposing the actual track borders or create gaps under and next to it. Those gaps that appear cannot be filled in afterwards because of intersecting terrain, so the best way is to get rid of that track section, raise or flatten the terrain, place the section again, and prey the result looks somewhat acceptable this time. Do this in small sections to stay close to the layout you were going for and hope that the auto complete function holds up.
I finally created a Pirate themed rapids that I am somewhat happy with, but it was quite a pain to get rid of the jaggy edges and I ended up covering half of the ride borders in rocks that sometimes do and occasionally don't allow to intersect with the ride.
And yes, rapids are following the tracks in an sterile way without any random factor or surprise movement. I suspect that is a limitation in the game engine since all guest also seem to follow the shortest route to a point of interest without wandering off or just exploring the park randomly. I also hope the go-karts AI will introduce more randomness to other aspect of the game.
