We had 8630 on the Fantasy and my DH didn't want to fight the crowds to see the fireworks, so he stayed in the room. He said he could see most of the fireworks from the balcony, but you have to understand, they go right up just above and to the side. So he said there were parts he couldn't see.
If you were standing on deck watching the fireworks, you'd have your head tilted, I'm guessing to an 80 degree angle from standing to see the majority of the show. So still enough to see from a balcony, but much better sightline on deck.