yes styrofoam / polystyrene is a know endocrine disruptor but just somethings here:
I've used polystyrene in making rock wall backgrounds, but when i use it it is completely covered (front, back, sides, etc) with something. Several layers of grout, non toxic acrylic paint, non toxic shields all sealer, and then silicone. Now can the endocrine disruptors break through all these barriers? I think the risk is more an issue with exposed polystyrene but once you have all these barriers I personally think the risk is much much much lower (i'm no scientist but i'm just going with the idea that silicone is used in tanks to seal them from water leaking so wouldn't it seal the polystyrene as well, and not to mention that its also covered in several layers of grout, acrylic paint, and shields all. Now of course after awhile silicone could dry out and rot but then the shields all barrier is still there, paint and grout.
As mentioned the safest thing is to not use it, but lots of people do, i think the important part if you use it is the steps you take to reduce the risk of any issues occurring.
Now also depending on what you are actually trying to build GS method can be easier and take less time to construct backgrounds. I know when i make a rockwall fixture from polystyrene it takes many many hours of me constructing it not to mention the added wait time of each grout layer drying, paint drying, shields all drying, silicone curing.
I'm getting ready to do my first mostly GS background now, but i used polystyrene again as the framework but it will just about all be covered over in GS then silicone and peat / coco mix. I don't like the idea of spraying GS directly on the glass of my vivs as in the removal of it down the line if i need to. I make my backgrounds so they are easily removable if so needed so i used the polystyrene in this build as the framework. Also i like being able to create the background outside of the tank especially when working with a fish tank where the only access is through the top. The idea of tyring to carve a background while its in the tank seems like a headache to me. Eggcrate would be another possible framework i could have used but i didn't feel like going to the store to get it as i have alot of polystyrene on hand. I'm also wondering how well spraying GS onto something like wax paper would work. If i can find some to build a framework out of that the GS doesn't stick to i'll probably do that on the next build