There have been incidents near me where a shipment of it was found to be contaminated. I don't know what with, but I remember going to the LFS and they were taking all the IO off the shelf. They told me that they had a few customers complain that they added it and there tanks crashed. Mind you, a few of these customers were not newbies by any means.(The poor assistant almost freaked when I asked him about it. He thought my tank was one of the ones that crashed. Thankfully I don't buy IO so I didn't have that problem)
The LFS became suspicious and mixed some of it for use and sure enough when they added it to an experimental tank their fish started going into shock. (Yes they quickly removed the fish to a recovery tank)
Beyond that freak accident, it also doesn't have as many nutrients and elements in it as the others. Which for those people who add supplements isn't really a problem, but for those that rely on a strict water change regime it is.
I've found that it's not as soluble as other salts used either. However, I used it about 2 years ago, so things could have changed.
I'm not trying to bash anyone who uses it, and hope that this post is not considered offensive. If you use it and it works for you then that is great. I just wanted to share my experience with it. 
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55gal,260 watt PC(2x65 actinic,2x65 10k daylight,8 lunar LED) 45#LR, 45#LS, 3 maxi-jet 900's on a natural wavemaker timer,phosban reactor, 5 gal fuge (built out of standard 10 gallon tank) with chaeto, grape, and feather caulerpa, sealife systems protein skimmer, wet/dry, 4 blue/green chromi, 1 skunk cleaner shrimp, 1 coral beauty,1 true perc, 5 mexican turbo snails, assorted dwarf blue leg hermits, astrea snails, and nerite snails |