While I have not used this product I can uderstand the rust and Cory issue. Many years ago many public works departments used coal cinders or bottom ash instead of salt and sand in the wintertime to apply to the streets for traction for autos and trucks. Many times while helping my dad at the public works garage (he was an employee) there would be huge chunks of the stuff. It would reminded me of the Texas holy rocks with the exception that it was black or brown and could also be multi color due to that fact it after all had been incinerated. I would also see chunks that had been there for some time and they were in fact rusted due to the iron or hematite content in the coal itself.
The fact that it is very jagged and sharp would again explain why cory's or other fish who would be face first in it could develope problems. I in fact did have problems with a "Java Sand" substrate early in my aquarium keeping. It was in fact a particular black sand that to my understanding was a type of volcanic slag and under a microscope you could easily see it was pointed cylindrical objects and my corys were getting redness around the mouth and face and they were not happy fish. In fact some did die and to this day I do feel it may have helped that happen. After I pulled the sand out and use small flint rock all the redness went away and they seemed to eat better and be happier fish. Coincidence ? Maybe, maybe not.