They are a real PITA for both the electricians and Plumbers.
The biggest problem is getting the rough-in in the right position. Even when the boxes are exactly placed with the template you are at the mercy of the plumbers for the exact location of the fixture. Fortunately this is really the plumber's equipment and it's his problem.....but it still can look bad when everything doesn't line up properly.
The power is 24vac. The transformer is provided with the flush kits. (plumbers equipment) The amp draw is negligible. Depending upon the size of the transformer, one transformer can power several flush valves.