Being a fast faller also makes you more easy to combo and can hurt your off stage game.
Meanwhile, Falco, Greninja, Sheik, and ZSS. And hell, let's add Duck Hunt?, Ganondorf, Ike, Meta Knight, Ryu, and Wario. Granted, Meta Knight has multiple jumps, but what do they have in common? All have fast fall speeds and all are considered good edgeguarders.
The problem with Roy's edgeguard game isn't his fall speed. It's the risk and reward you get from his moves. Forced to go way up close to edgeguard is really risky compared to having to space well like Marth. Oh, but what about Cloud? He's a fast faller and he has a ton of range. Aside from arguably the intentionally worst recovery in the game since even Little Mac can ledge sweet-spot, his aerials have too much recovery for him and by the time they end, he's screwed without meter letting him recover or letting him stall like with Limit Cross Slash which is an edgeguard option he uses.
Roy? To put it simply, he plays too much like a Melee character, which is really bad in this game. A fast faller with poor aerial mobility and an underwhelming recovery.
I'm sorry, but what? Roy's air speed is among the best. His air acceleration is 0.3 like Mewtwo's and just 0.1 below Ike's who's also considered to have good air speed, but bad air acceleration.
If you want a Melee character, then Cloud, Dr. Mario, Falco, and Little Mac are your answers. All of them have deliberately bad recoveries. Either they're just bad on purpose like Cloud and Little Mac's or inferior for the sake of being inferior to their counterparts like Dr. Mario and Falco's with stupid justifications like maybe Doctor Tornado which has lower overall distance gain than Luigi Cyclone is enough for the Doc to have a lower Super Jump Punch or Falco's jump height... Hey, remember how Greninja and ZSS are right below in jump height to Falco and they have superior air speed, air acceleration, and recovery moves? Cloud's the only one with good air speed, but his recovery makes Melee recoveries look good. Dr. Mario's not a fast faller, but his poor recovery moves in a game where 90% of the cast has good ones is pretty stupid. Falco just has a big: "Screw you!", stamped on him.
Roy at least can ledge sweet-spot, has a hitbox on the entirety of his recovery move, his recovery move connects properly, his Blazer isn't just a crappier, multi-hit version of Dolphin Slash, and his air speed isn't just bad for the sake of being bad like the Doc, Falco, and Little Mac's. When Roy gets hit off-stage, he can ledge sweet-spot so well when the others can be closer, but still fall out 'cause screw them having decent recovery moves with good ledge sweet-spotting. Roy's biggest problem for recovering is that Blazer travels slower than Dolphin Slash making it easier to time an edgeguard and because it's his only recovery move, he's got a very linear recovery, but he can angle his Blazer to try for mixups. That being said, if he reaches top air speed, he can choose to land on-stage.
Roy's problem in general is that... Like (Lucina and) Marth, they're not getting rewarded for their spacing. Marth had to be like a surgeon and Roy has to be in your face for kills. Other characters? They can space pretty much however they want and get the same rewards as them. It gets worse when you realize there's characters with the "same range" as them and they're just as or even faster and stronger. Meta Knight does up-close combst far better than Roy can hope for. Oh, Meta Knight's not as strong, raw power-wise, you say? Then how about the Pits? Cloud does spacing better than Marth when outside of some moves like Up Smash, Side Smash, and Fair, Cloud's as fast as Marth and moves faster than Marth while hitting harder. Oh, Cloud's not like Marth... How about the Pits, then? The same can be said about slower, two-handed swordsmen. Ike does what Shulk doesn't. None of them are the same character, but some of them outperform or under-perform when compared to each other.