They slow down the flow of water a bit but for areas that are somewhat steep that is probably more of a benefit than a drawback.

Slowing a river down raises the water level, which can risk flooding. Every section of river has to let through a certain amount of water per second, and slowing the water down means you need a bigger cross-sectional area to get the same throughput so the water level rises. This can create a feedback spiral because a higher water level slows the river down further because there is less height difference with further upstream and thus less force driving the water downstream against the friction of the riverbanks.

Luckily it seems these turbines can be removed quickly, so unless the engineers fuck up they shouldn’t increase flood risk.