Interesting you recently posted this. I have been using Faultless premium for years and there has been very little build up on my iron (none noticed). Because I was using fusible recently I just cleaned my iron. Today I tried Niagra in the manual spray bottle. All I did was iron 300" of binding in half - I use starch when I do this to hold the fabric together while I iron it.
When I looked at the bottom of my iron it was horrible. the only part of the iron without gunk was where I actually iron the the binding in the middle of the iron.
I recently added a Steady Betty ironing board cover, but I don't think that has anything to do with it.
I suspect different starches act differently.