After playing more and more Sonic I still don't find his spring to be broken. If you're playing a good opponent, they'll be expecting you to use it when they recover, and they will recover accordingly. Very rarely is your opponent without options in their recovery. They can do things like recover fast (before you get out past the ledge, usually very possible), double jump airdodge, recover high (above you), or, what is likely most possible for all characters,
simply hit the spring with an attack. The spring can be canceled about by what seems to be almost any move. A simple uair by most characters will be more than enough to cancel it out, and hell, a lot of characters' up-b moves themselves will cancel it out. So, not broken, your friends just need to get up on their recovery game
Without the spring I think Sonic wouldn't be too hot (as it is still possible to hit with using some good mindgames, or in situations where your opponent is pretty far away from the ledge and doesn't have much flexibility, or when your opponent's recovery just plain isn't that good)