There’s nothing wrong with magnesium chloride per se when it comes to creating the brine. The biggest issues are cost and interference with the CH test. Magnesium has high solubility in water similar to sodium and potassium. It usually will only scale once the pH is above 10 (which can happen INSIDE and SWG cell

).
Magnesium sulfate (Epsom salt) is bad because the sulfates added by it are not good for pool plaster and SWG cells. People love to soak in Epsom salt because of skin and joint issues but it makes very little sense to do that in a swimming pool.
Pool chemical suppliers will say whatever they can get away with to sell a bag of salt for more than it’s worth. Here in the USA it’s not uncommon to find bags of cyanuric acid labeled with “Specially Formulated for Salt Pools” and they charge 1.5X the price of the “boring” CYA

. There’s literally no difference between the two.