The only risk is that water picks up minerals passing through the earth. The ideal (from what I've read) is 150ppm (+/- 50ppm). If rainwater is pure, like reverse-osmosis water, you could have problems regarding trace minerals. The soil should be sufficient for that. But, you never know.
If your tap water has very high ppm (like mine), that can be a problem too. I mix about 30% tap to 70% ro water (to get around 150ppm).
I've seen people here use it. It seems good. The problem is that you have to be prepared to feed after 2+ months. (It says 3 months, but it doesn't seem like it makes it that long). People tend to not realize this and have deficiencies at 3 months.
I've seen people here use it. It seems good. The problem is that you have to be prepared to feed after 2+ months. (It says 3 months, but it doesn't seem like it makes it that long). People tend to not realize this an have deficiencies at 3 months.