I am a goalkeeper myself (in football though). I would 100% agree that specific and indvidualized goalkeeper trainign is one of the most important and best things you can do if you wan tto develop goalkeepers. There are certainly a lot of attributs a great goalkeeper should have. I think that a handball goalie needs to have great reflexes but also a good timing and flexibility. All of these things can be very well trained with specific goalie practice.

I would say that that a goalie is more than 50% of a team as he is the last line of the defence. If he plays well you have at least a draw and if he plays very bad you lose no matter what :D