I work on fish passage projects for a living. (Engineer).
It is true that fish ladders can be added/incorporated into artificial river features. But - it's not entirely straightforward and is almost never as good as a natural river. There is just more variation for the different ways different types of fish move in a natural river. (Think small eddies that fish rest in, think about how some fish are jumpers, some fish are strong swimmers and some fish use there little suckers to cling onto something, rest and then whip their tails to propel themselves up to the next boulder). Because fish ladders cost money, engineers are trying to economize it as much as possible - which means we sometimes are designing it for one species rather than all species.
While there are in most rivers, obstacles that seem as difficult as an artificial river may be, a few other things to keep in mind -
Sometimes fish only make it above a particular feature during a particular water flow - and it is an obstacle at other flows (think of washed out drops). It isn't great for fish if they get the flow they need to get past one obstacle, but then are stopped by a new obstacle.
Also every obstacle that a fish has to overcome, expends a lot of energy. The more energy that gets used up, the shorter the distance they are moving upstream. This means they may not reach the habitat they need, or the habitat may be more crowded.