I consider salt brand selection as dealing with a known evil. It's easy for us to use the salt brand as a scape goat for the many unknowns out there when our tanks go awry. Salt often takes the fall for vibrio (bacterial infections), reef parasites such as acropora eating flatworms (AEFW) and montipora eating nudibranchs (MEN), as well as secondary metabolites (coral toxins). These are hard conditions to diagnose and even harder to treat, but changing salt is simple, and more importantly "somebody else's fault"
I'm not suggesting for a second that salt mixes are not to blame for downturns and tank crashes, these can and do happen. Some people change salt brands annually to provide more varied and balanced water chemistry. Perhaps that one loose end parameter such as iodide or potassium will fall in line with a brand change. Perhaps your current salt mix has a surplus of one particular element that your tank inhabitants don't readily utilize; a brand change will likely amend this issue.
With respect to Instant Ocean products, standard IO is best suited for fish only tanks as the calcium level is only 350-400 at 35ppt which is below NSW (natural sea water) levels. The KH of IO and IORC is the same (3.8-4.2, 11-12 @ 35ppt) which is higher than NSW (8). The magnesium level of IO and IORC is the same (1400 @ 35ppt) which is also higher than NSW (1350).
The calcium level of Instant Ocean Reef Crystals (IORC) is 450-500 which is way above NSW. If you were to use IORC in a lightly stocked reef tank and dosed or used a calcium reactor, the calcium level would likely rise to unsafe levels (>450). It is not uncommon for a new hobbyist to blindly dose two part additives without testing.
Some people use IO for reef tanks and test and amend the calcium to 400-430ppm. I have had a few reef tanks drift up to 500ppm due to the high levels in IORC.
I know a few people who stopped using H2 Ocean because the alk of freshly mixed water at 35ppt was 14 dkh. The high alk mixed with calcium reactors and chemical dosing ended up driving their KH to dangerous levels (>14). Apparently, this problem has been amended with the new H2 Ocean mix. A high Kh isn't always a problem. It is entirely possible that a nuisance algae problem could be eradicated with a KH of 12 or greater.
This brings me back to the known evil theory. I have used IO for 34 years. I'm currently using IO for fish only tanks and IORC for reef tanks. I cannot say with any certainty that I have ever had a salt mix related problem, other than high calcium from IORC. That's good enough for me
I also look at what others are using and IORC seems to be the most common brand for public aquaria and large scale reef tanks such as Chingchai's in Thailand.