Guess Ill Chime in here.
My only real concerns would be litter left in the field. As pointed out, this design works on the premis of it striking something, breaking open and spraying bb's via the force put into it from the throw, and the sudden impact of the hard surface.
that being said.
1 You are not allowed to trow things directly at people
2 you are not allowed to overhand a grenade when throwing it at someone.
3 Plastic shrapnel from a egg that has been thrown with force isnt a good thing
4 lastly. Litter, Did i already say litter? well, Im saying it again. Yeah you might say you will pick them up, and even the pieces of the shatterd ones.. but that dont last, You can already find spend nade shells all over the field as it is.
