I looked into it, Devils own has some nice kit, the one potential scare story was malfunctioning controllers which meant the pump ran when the engine was not running, basically in this scenario the entire tank of water dumps into the intake. Obviously not great come startup with a inlet full of water.
The Devils own stuff sounds good but is relatively simple, just runs off boost which means when you hit the rev limiter, it continues to pump water/meth even if injector or timing has cut. Meth itself is combustible, so your engine is running super lean on just meth (obviously not good). I am not sure how the rev limiter comes in on the MKV.
I looked into Aquamist systems, these have more safety measures, they hook into a injector signal and only allow water injection when the engine is running and injecting fuel. They also calculate the amount of water to inject based on fuel injection rate and boost. Overall a safer, more complex but more costly system.
I was really keen to go water/meth for the ability to run more timing etc, but wimped out the more I read into it !