I think there is also an issue of misleading data. Take the link to the company talking about too much magnesium in the diet.
Without analysing hay and grass which are the main sources of food for most horse the company will not be able to know exactly what levels of magnesium is on the diet. The data is based on what individual horses owners are telling them they feed. The only way as far as I am aware to analysis magnesium levels in horses is via blood test. So for the data to be correct that 7000 horses had too much magnesium in their diet and then after taking their supplements their magnesium levels reduced resulting in improved behaviour to be scientifically valid would have required blood tests before and after supplementation to show magnesium levels had dropped.
Their supplements are based on calcium and in people magnesium and calcium levels seems to be linked. So if a horse has too much mag then it may well have high levels of calcium too. Adding a supplement which is based on calcium when it already has enough calcium in the diet can lead to calcium overdoses.
A lot of these companies will have scientists working for them who must surely understand the basics of scientific data and research. It is fine to say we have happy customers who have used this product on x number of horses and have said the have noticed improvements, {if of course it is true} but not fine to say that this is scientific proof the product works in the way it is described as working.
Many companies will have based their products on human research as there is more money and academics working on humans than horses. When I buy supplements I try and do a bit of research first on what has been done with people first.
Unfortunately this is yet another gaping wide analysis of how the equestrian market is left vulnerable by a large gap in knowledge - one so wide you have even professionals preying on the ignorant mass of consumers willing to buy anything that will miraculously transform dobbin into pegasus.
The Magic Supplement - no one wants to hear that they actually need to train their horse.
My other favourite is the Cure All usually touted on the "Natural" sites: like Tumeric! Cures everything don't you know from arthritis to cancer. Apple cider vinegar and garlic are a couple of others that have done the rounds, but now seem to be out of fashion.