I'm reading Dave Miller's Brew Like a Pro and one of the items he states that would make water unbrewable (basically saying if you have this go with RO water) are iron levels anywhere near the threshold level of 0.3 ppm. He states the acceptable limit of iron (not sure if it was a typo or the figure he actually meant as opposed to the threshold level) at 0.03 ppm for brewing water. According to my city's water report, the iron is at 0.3 ppm, well above what Miller said is the limit. He doesn't explain why iron makes the water unbrewable, though. Is it just that it will impart a strong iron taste? I don't believe I have detected this in my beer, but maybe my palate just sucks. My full water report is below if it makes a difference.
Water Report Parts Per Million (PPM)
Total Hardness 90
Total Alkalinity 35
Phosphate, Tot 0.3
Phosphate, Ortho 0.2