fly. Some but not many. With made goals, there is going to be a stoppage in play just like if the ball went out of bounds because there will be the need to report scores, check sticks, etc. that goes on after a typical score.
If you make subbing only on the fly, the coach is going to have his players run to the substitution box and just swap out on the whistle with his defensive guys. The ball is more than likely going to be placed either on the side near the endline, behind the goal with the goalie or with the goalie in the crease.
It was tried before and it failed to do what people think it will do.