Keep in mind that any bucket (or box) of salt MUST be mixed before using it.
When these salts are 'manufactured', or prepared for sale, they come from an assembly line that fills the bucket with components one at a time. So when they fill (just for instance here) 50% calcium then 20% magnesium then 20% alkalinity then 10% trace elements, those components can easily be separated in the bucket. Therefore, when you use the top portion of the bucket, you might get more magnesium than your mix should contain. When you get to the bottom there might be a higher concentration of calcium than at the top. This is why it is very important to mix the bucket to get a homogeneous mixture, unless of course you're mixing the entire bucket at one time.
This one feature of buckets of salt vs ESV and its requirement to personally mix your own salt mix makes the ESV salt more consistent from mix to mix. IMO that's a very important and positive feature of ESV.
Quite frankly, every salt out there probably does its job and will sustain your reef, some just better than others.
Russ