Well, I've been learning sephiroth exclusively since he launched as he looked interesting(not an ff7 kid) to play as, and it feels to me like he's just a better Ridley. After 2 years of maining Ridley I think I'm ready to drop him and just keep him as my fun character. I play samus and wolf on the side, plus sephiroth now and neither of those characters has to deal with all the junk that ridley does.
I could elaborate on why sephi is a better rids but most of it feels obvious? Getting the real obvious things out of the way, Dair. Come one, what the ** is that about? They just gave him the same move but that actually works, and really well. When you have Wing, your'e flat out better at what I perceive to be Ridley's own game. Incredible edge guarding, great advantage when below the opponent, high damaging(or well, insta kill) setups....you name it. Not really complaining, I really enjoy playing sephiroth but sakurai really just ** on ridley's chest this time around ¯\(ツ)/¯
I really hyped Seph too at first, but it's not as black and white as "Sephiroth good, Ridley bad", even with the better d-air. Since we've gone over Ridley's weaknesses to death, let's go into detail over Sephiroth's to make it fair.
Everything besides his range/power is lackluster. Bad frame data, is super lightweight (similar to Mewtwo as a glass cannon), recovery that is easily abused and exploited in a similar vein to Ridley's...
As much as his range is hyped, it's also not even that good on many of his moves. His sword is like a poking stick that has extreme range in one direction, but is easily crossed up or dodged (plus you know, the whiffing). An example on his range, his up smash is way too slow and precise to be as good as say Ganondorf's up smash. Sephiroth's f-smash whiffs all the time as foes run past him during its gigantic start up, and his down smash is just for hard reads more or less, like Ridley's. His coverage is bad despite the range.
He has to rely heavily on Flare to the point reflectors and absorbers are a huge problem. If his projectiles were disjointed it'd be better. I will say though, Gigaflare is legitimately giant enough to be difficult to get around at ledge, but requires you hit the foe pretty far off stage for it to matter.
Like Ridley, he gets zoned by anyone with a half decent projectile. Has no response to that or reflectors, as Gigaflare is so important to his play.
His down special is honestly the worst counter in the game, which is unfortunate with how required he is to use it in most matchups. You know something's wrong when Ridley's Skewer can hit him before his counter comes out (what happens when their down specials are done at the same time), and since it's technically a projectile, if you counter something like Ness/Lucas' f-smash, you basically just die... The counter also directly loses to multihit projectiles and hit him, on top of breaking against strong projectiles. Plus that break in the middle for foes to just hit him out of it if they wait to like charge a smash, which Ridley can do and break his counter with as pointed out earlier.
His OWA doesn't do enough for him I'd argue; not like Arsene does for Joker. That said, his KO power is reliant on that (OWA affecting his slow smashes with the armor) and his tipper, as he doesn't have KO throws and such.
The problem with Sephiroth mostly comes down to how hard he loses neutral to anyone fast with combo moves and in disadvantage there's very little he can do like with Ridley. You have to just stomp the opposing player into dust like Ganondorf or Mac. He certainly isn't winning just off his slow and telegraphed melee game. He's like Mac in the sense that he has massive power but dies very quickly.
This isn't just me saying this kind of stuff btw. Even Seph's Discord was basically like "He-he's totally high tier! Er, maybe mid... H-haha...", on first impression.
You do you though on who you main or secondary. I don't blame you or anyone who drops Rid down to a secondary personally, but just keeping it honest here. Seph could use buffs like Ridley as it is with all that said, though may not happen with the diminishing character adjustments that have been had with the updates as of late.
The dair is already easy to punish when it hits onstage, because it doesn't trade with hardly any moves. Since the hitbox isn't even shifted below Ridley's feet, it's a free punish against him if he uses it. Even the double jump offstage setup doesn't guarantee a spike, and when it doesn't spike, you just kill yourself since you used up the jumps. Yes, there is downward Wing Blitz, but you can't expect to always spike with it in a pinch with the startup that's required. 
: Get on it, Sakurai!