I have tried GFO a few times and got dead clams and STN for it. For some reason my tank is very sensitive to excess iron levels and GFO is a no-go for me. I have not had issues with keeping phosphates low even without it, I have used various carbon dosing systems with varying degrees of success but I have been very stable now for the better part of a year with NP biopellets and now All-in-one biopellets. If the all-in-one pellets contain some sort of GFO it gets skimmed out immediately.
GFO can easily be overdosed, and other than the "Too low nutrient" issues others have spoke about here, too much Iron can trigger STN in a carbon dosed tank in my experience.
My tank - All in one biopellets and a big skimmer only: