With normal feeding artemia are a decent source of protein, the source being the "meat" part of the shrimp itself. Gut loading the shrimp means that it has this nutrient
PLUS what ever you use to gut load it, the extra coming from the actual contents of the digestive tract containing what ever you gut load it.
You can gut load with more protein, with lipids, or with medications, whatever suits your application best.
Much of the controversy around nutrition is as WaterKeeper mentioned, but also, because people read the protein content of frozen brine shrimp which unfortunately for the most part is a percentage based on total package content, including packing fluids and shrimp water content.
"MOST" other foods list protein content based on dry weight percentages and thus make comparison misleading.
Now, if you take that flake food like spirulina that I and some others feed, and wet it in the tank, and
THEN calculate percentages based on wet weight, and take thawed, rinsed frozen brine and calculate wet weight percentages, you will find the brine shrimp protein to be higher than the spirulina.
While protein levels are decent with artemia, mysis shrimp are slightly better, based on similar dry or wet weight percent. However, culturing mysis in any numbers similar to numbers in an artemia culture are next to economically impossible.
The mysis are cannibalistic, take months to grow, and don't multiply anywhere near the rate that artemia do.
For nutrient levels comparisons of GSL brine shrimp see the section 4.4.1 of the article written by the Artemia Reference Center at the University of Ghent and posted on the United Nations Fisheries and Oceans website.
Nutritional properties of ongrown Artemia