A perhaps bigger drawback is that the foundation you apply the gesso to must be rigid. Even the 1/8" hardboard may not be rigid enough to keep the gesso from cracking if the board should flex. When I painted the below painting, even though it was on an 8x10 panel, which shouldn't flex much, I detected cracks. Oddly, I couldn't see the cracks when the panel was dry, but when I applied my initial underpainting, I could see cracks quite clearly.

I sanded the surface lightly with fine steel wool prior to painting. The sanding didn't remove the marble dust texture, which is a good thing.
The solution to these two headaches is, of course, acrylic gesso. You don't have to heat it and it won't crack because the acrylic forms a somewhat flexible film.
Again, as with the Gessobord, this surface is very absorbent. A coat of medium prior to painting would be ideal.
(And still, not a bad painting!)