Dancing Blade, what I would call his best move right now, is ruined for Ultimate. It's still okay – being a transcendent priority, disjointed multihit, but it's seen extreme nerfs. It was sped up probably so that each hit does less hitstun (knockback) and to justify damage nerfs. These were done to nerf the strategy of slowing DB down to trick shielding opponents and to wait for the tipper AND give us time to think and read DI. Sakurai no doubt also wanted to make it easier on casual players that just want to mash side B. Wouldn't work in all Smash games except Ultimate seeing how DB goes 2x faster now. In addition to all that, you got reduced kill power and I saw characters falling out of it in many gameplay videos – in that you're punished for getting the tip. It's partly because Marth isn't walking forward anymore in the animations. He kind of stays in place.
But then, this startup nerf is more significant than all that other stuff combined. It's 1.5x slower on a move that is supposed to be both an out of shield option (frame 6 out of shield drop can punish many things; frame 9... can't punish much, especially given the landing lag reductions and shield stun increase Ultimate has) and supposed to be Marth's dash attack (we all know his real dash attack sucks). If you want to know what F9 DB is like: use the Heavy Blade custom side B.
It's funny, the startup is nerfed on not just the first hit... it's nerfed on all hits. Hits 2 to 4 have severe startup reductions, even though the total frames of each hit are the same. So if you find yourself DBing a shield (which you will more often now that it's F9) and you stop the combo (whether it's at hit 2, 3, 4), your opponent will have more time to punish you. And forget about getting that follow-up tilt or aerial (into potential combo or zero-to-death) after DB4-up at low percent... there's more lag now. The DB4-down multihit may even be so fast that it locks the opponent in shield even if they release the trigger. Say goodbye to them dropping shield before the last hit. And because the last hitbox ends about 20 frames before it did in Smash 4 (crazy right!?), they have so MUCH TIME to punish you that they'll be half-charging their strongest Smash attack right in your face.
Outside of competitive, you know what sucks? To enable the "speed increase", they heavily simplified the animation. It no longer looks like a dance at all! Just a bunch of left and right swipes. Even worse: Marth, Lucina, Roy, and Chrom all use the same DB animations. So far: Melee, Brawl, Smash 4, and Ultimate all use different DB animations from each other. How difficult would it be for them to differentiate those four types into each of those four characters? It's tiring to see all the Fire Emblem characters use the same moves.
Speaking of left and right swipes, going back to competitive, where's the coverage on it now? They were arching hitboxes (even DB1 and DB3-forward that were supposed to be "forward" hits) that had great anti-air capabilities. Not anymore.
