Well, They are really smooth so they won't provide a lot for the roots, solids, bio-slime or bacteria to cling to but that is only minor.
The bigger issue is if they are transparent glass marbles, they will let light down into the grow bed and allow more algae growth and not promote as much bacteria or root growth up nearer the surface layers. That light may also be magnified to points and cause some minor increased heating with sun shining on them.
I expect they would be a more appropriate media in the bottom of a display aquarium rather than the top of a grow bed.
i tried chipped up beer bottles, sifted through a fine screen, it wouldnt cut your hand if you squeezed it. It worked and grew great tomatoes, the only thing was the PH of the glass was high, and heat eventually killed the plants. I sent some off to the county extension for testing, there were alot of chemicals in the glass, and PH was crazy, so i went to pea gravel. It is alot cheaper to goto a concrete business that makes their own, than buying it from a landscape place.
i dont see a need for hydroton, its too much money, ive got some in a tiny system i just built sunday, a 6 liter indoor system above the crayfish tank, i do not see how it is worth the extra money
They Hydroton only starts to be cost effective if you are also figuring paying other people as the labor for washing gravel. If you can take a little excersize and wash your own gravel or have a little party and get it done with a group of people then Hydroton only becomes necessary where weight is a major concern. The Hydroton is easier to plant transplants into since when the bed is flooded the media will move easily so you can almost swish a plant in while gravel requires digging since it is heavy. However, heavy media is helpful for holding up tall or large plants (corn, etc.)
Bear in mind that Pea gravel is not all the same!!!!!! Test your gravel before buying a truck load since limestone pea gravel won't be good for aquaponics. You need something that won't mess with pH too much.