Hello WezV ! As far as I am aware, Titebond III is perfect acceptable for use in guitar making as far as strength goes. It will hold up as well Titebond original, if not better. I believe the reason most people here don't use it is that it is designed for marine use I think, and therefore is pretty much water proof. So, if you ever had to steam a joint loose to, for example, replace a broken set neck, or change out a fretboard, you would probably have a devil of a time doing it.
Anyone feel free to correct me if I am wrong, but I used Titebond III on my first build and had no problems with it. I have since used Titebond original and no problems there either.