There have been many studies done over the years showing not only the leaching of phosphate once the media is exhausted but also the release of aluminum. Here is one such study:
Chemistry And The Aquarium: Aluminum In The Reef Aquarium ? Advanced Aquarist | Aquarist Magazine and Blog

I know Seachem did a paper on this but it was performed in house and not by a third party so is of limited value in my mind since they have a stake in the outcome.

Having worked in the municipal water treatment industry for going on 40 years now I know ferric oxide is the preferred media for arsenic removal and has been proven for decades. GFO and GFH once exhausted just don't adsorb anymore but they do not release and is why they are EPA and ANSI/NSF approved for human drinking water It is where we as aquarists have taken both of these medias from originally. There are also newer medias such as coated titanium dioxide that stands up better than GFO. I will continue to use a combination of GFO and a refugium with macroalgaes until something better comes along. Another perk of working in the industry is I have sources for less expensive GFO which helps out too.

Here are more studies and papers and none shows any release of substances once GFO is exhausted. A couple point out the advantage of a slower reaction than aluminum based medias and its effects on corals.

Iron Oxide Hydroxide (GFO) Phosphate Binders by Randy Holmes-Farley - Reefkeeping.com

Phosphate and the Reef Aquarium by Randy Holmes-Farley - Reefkeeping.com

Aquarium Chemistry: Effects of GFO (Granular Ferric Oxide) on 'Trace' Metals Concentrations in Artificial Seawater ? Advanced Aquarist | Aquarist Magazine and Blog


I guess it boils down to it being a personal choice as there are supporters of both medias as well as more recent articles on Lanthanum Chloride but I don't think I'm ready to go there yet.