Yes, the problem is that the changeover moment between 'tons of water moving uphill' and 'tons of water moving downhill' is inviting a serious case of water hammer if not managed properly. Typically it takes about 30 seconds for things to first quiet down and then to allow the flow to reverse. The turbines themselves are already spun up during that interval, either with compressed air or by temporarily using the generators as unloaded motors so they're already synchronized to the grid. The valves are then opened gradually to the point that the generators start leading the phases and hence put power into the grid.

It's fascinating how mechanical all of this is.

When stuff goes wrong with dams they really go wrong:

https://en.wikipedia.org/wiki/Sayano-Shushenskaya_power_stat...

Standing inside the turbine hall of one of these you can't help but think of the absolutely insane pressure from a water column typically 100's of meters high and half a meter or more across pushing down on a fairly simple valve. Engineering at its finest. To me the big dams are right up there with spaceflight.