I cannot speak to other individuals results, just my own. I have used Special Kitty (the cheap stuff from Walmart) for about 10 years in my emersed 'plant bank' as the topping layer in my pots. After a year it still retains most of its granular integrity, difficult to break up a grain. After two years it is softer and the grain breaks more easily. After three years the I can break a grain between my thumb and forefinger. These are grains of Special Kitty that are not submerged however subject to very, very high humidity. On 02/27/14 I set up my 75 gallon with STS. It ran until 11/27/17 when I did a complete break-down; about 3-3/4 years. At that time the STS was about as soft as the Special Kitty is at three years.
Regrettably the calcined Montmorillonite clay material I like best is no longer available (at least here on the West Coast). It was Soilmaster Select Charcoal colored. It is still going strong in tanks 9 years old. Yes, the higher the CEC of a substrate the more it will lower the pH by stripping out the carbonate ions. Actually I believe most of the nutrient ion exchange occurs with the roots on the surface of the grains of substrate, not within the substrate.
My emersed 'Plant Bank' with humidomes
You can see the Special Kitty as the top layer in this picture.