I have had quite a bit of the current FAB powder. I cannot compare it to the Mexican detergents as I have no experience there, but I was generally impressed with FAB's whitening ability and its low sudsing properties. I have never seen any problems with it dissolving, but I never wash in cold or even cool water unless I use ColdWater Tide. Charlotte city tap water is fairly soft, but not extremely so. My folks have a water softener on their well, so low sudsing detergents are important there, and FAB has been good in both applications.
My laundry is only occasionally really challenging (automotive work, working on washers, or working in the yard) but I have been happiest with Tide and Cheer. FAB is less than half the price of those products in general, so you probably at least partially get what you pay for, though I question if Tide is worth double the price.
One of our stores here puts FAB on BOGO, which it is right now. I would rather have Tide on sale, then use a coupon which the store doubles.
Gordon