AWS A5.20 restricts their use to 3/4 inch or less. The manufacturer often restricts their particular product to something less than 3/4. It all has to do with chemistry, i.e., the flux constituents and how they interact with the molten weld puddle. Strong deoxidizers often act as alloying constituents if they are not needed for the purpose of deoxidation. The excess can cause strong welds with little ductility or they can adversely affect toughness. Neither property degradation is desirable.

Best regards - Al