The biggest problem for rain water harvesting is there is still no pressure for harvesting rainwater, because in the most areas we have enough water. But there are some areas which have less rain and are lacking rain water in some time of the year. And quality can become an issue. If the drinkling water which is delivered by pipes in good quality will become more polluted by NO, N2O and micropollutants raising the costs of drinking water production it might be a possibilitiy for using more rain water harvesting.
Another general problem is the discontinous rainfall providing the water to be collected. This leads to costs for the storages and pumps which are higher compared to dirinkling water prices from pipes.
Instead of rain water harvestiing the harvesting and clarisfication of grey water could be a better solution for the continous filling of storages and emptyiung by continous usage. Here we are facing the similar problem of costs, even the technology is ready for the market.