Hydro is pretty great with a few caveats. Flooding remain a massive problem if there is a failure, as the world most deadliest power plant accident was a hydro power plant with a death toll between 26,000 to 240,000 people. An other major issue is the extinction of species, as is currently occurring in Europe and especially in the northern parts. There are technologies to allow for migrating fish to bypass hydro power plants without chopping them to bits, through the effectiveness of the bypass tend to work against the power output of the plant.