Hm...
Roy doesn't have a whole lot of confirmed true combos, I think, if any. He has a couple of nice mixups, but he can't rack up damage that efficiently. He also doesn't really have all that many options from his grabs - Marth and Lucina admittely don't have many either.
Roy has way too many inherent design flaws that are keeping him from being better. First, his jumps aren't very good, especially his shorthop. His air acceleration is kind of bad which means he takes a while to actually get going in the air. Even if you don't miss this during a fight, you will miss this while recovering, which is another weakness of his, since his Up Special doesn't cover that much distance.
He is also a lightweight fastfaller, which makes him combo food. But unlike many other lightweight-fastfallers in this game such as Fox, he can't really get out of combos as reliably as them, which means he ends up soaking too much damage himself, and his iffy recovery gives him average survivability at best. Atleast being a fast faller makes him harder to KO off of the top, as I've actually seen Macs not KOing him with KO Punch at 50% (though that's with pinpoint DI).
The Hilt mechanic also kind of goes against what a disjointed sword is supposed to do: It forces Roy, a character that can't even approach that well, to go balls to the wall against his target simply because he can't take much advantage of it. A lot like Marth, his non-sweetspotted hits are nothing but a slap, but much unlike Marth, he can't get much from those hits. He needs to get up close and personal with his sword, meaning he can't take much advantage of his disjointed range (which is rather poor to begin with) and his frame data cannot really accomodate such a reckless playstyle to begin with.
Sadly, Roy just kind of got the short end of the stick in Smash 4 again.