The problem with playing goalie is that everyone else on the ice has a lot more control over their players and hockey sticks than you do. You basically have a limited amount of moves/animations to stop an almost unlimited array of scoring situations.
That, and often it feels like you're nearly helpless on goals against. Not only do you have the old backdoor-one-timer which is almost impossible to stop, but sometimes it seems like even mundane shots go in the net because the CPU decided it was time to let one in. I don't even know how many times I've been square to a shot, tracking it fine, and it just zips past my glove into the net.
I guess it's necessary to let shooters win the battle sometimes, due to attribute superiority and what not, but it's just not fun to play a game where you can only marginally influence the outcome. It's totally rage-inducing to be in the middle of a solid game and then watch your goalie wave at two or three long wrist shots. Makes me wonder why I wasted the time playing the first part of the game if the outcome was going to be determined like that.
Like you said, it comes down to not really having full control over the goalie. I'm not even sure if it's something that can be fixed, given the limitations of a video game controller. For now, the whole mode is just very unrewarding of the time and effort it takes to build your skills to the point of keeping your head above water.