Crescent Slash is better, no doubt. Even if DS had fixed hitboxes. You have a combo or at least a set-up from: Jab 2, Dash Attack, Ftilt, Nair, landing Nair, Fair, landing Fair, Bair, landing Bair, Fthrow, Bthrow, and Dashing Assault. And it still has it's combo/jab breaking uses with the frame 1 invincibility.
This is basically a Marth version of Bouncing Fish.
And on Battlefield you can almost use it punish-free and cover a huge portion of the stage while nearly wavelanding on the side platforms (and Smashville's is basically a true waveland).
If the Fox and Marth are on the same skill level the Marth shouldn't be too worried. I now admit Marth loses, but it's not by much.
Shield Breaker is so misused. I get pissed going against those Marth's on FG that think the ONLY way to land is landing with Shield Breaker. They spam it so hard that they literally run away JUST so they can turn around, jump, and charge it. It's release time is fast and it doesn't lag too hard so you sometimes just have to let them do it.
It has so much potential as an occasional say surprise ledge get-up read > they shield > you break or to punish shielding after end lag or for those times you have someone on a platform or an early partial-charged kill near the ledge.
And when spaced, it's barely punishable on the shields it doesn't break.
But no, some of them just spam it instead of keeping it locked away for the right moment to do something smart with it.