I haven't used it and didn't find much about it's use by others in my short search. I did find the Material Safety Data Sheet (MSDS).
From this site near the bottom of the page under 'Water-based Deck Finish':
SoyGuardÃ‚Â® Premium Water Repellant & Wood Sealer is mostly natural, but contains 19% recycled polystyrene. SoySeal Wood Sealer & Waterproofer is a natural soy based sealer for all wood, decks and docks. It provides a 24-month seal that spreads water and doesn't bead. It contains no V.O.C's, is non-toxic, non-flammable, and biodegradable.
At another site it mentions that Iowa State University Extension tested it and said it was safe. A search of Iowa State Univ Extension didn't turn up anything on it. When I searched the entire Univ site I found SoyGurad as a combo of two products used to treat seeds to prevent fungal problems.
Hope someone else can give you more info.