A few people on here have used these and reported great success with them as long as the cartridges are regularly renewed. Some have seen some impact on pH, but as your kH is very high you're unlikely to see that effect.
RO filters remove everything from the water, not just nitrate. I use RO myself (I also have high nitrate in my supply), but that's because I keep a lot of soft water fish so need to reduce the mineral content of the water as well. The advantages of RO are that you can tailor the water very specifically for different fish by adding back varying amounts of minerals and buffers and you no longer need to use a dechlorinator as they also remove chlorine and chloramines. On the downside, production is slow