HIKARU used DK vs. Ly's Byleth. Also, Zackray went all Sheik in Grand Finals. Seemed more like he was practicing setups than actually playing the game at some points, so I wouldn't be surprised if Zackray considered this a level-up event for his Sheik.
It seems that a few Japanese players are trying out Byleth, which will hopefully give us more good data on the character.
One thing to be aware of is that Leo and sparg0 aren't solo'ing their mains. When Leo runs into a bad Byleth matchup, like Pikachu, Sonic, Min Min, and now Aegis, he'll consider a swap. sparg0 will swap to Cloud for many zoners so we don't get to see Aegis vs. zoners like Samus and Pacman. Thankfully, Cosmos shows us that side of Aegis' matchups, but we don't get similar top-level data for Byleth.
In the case of other high/top tiers, like Pikachu and ZSS and ROB, we see their mains fighting some really bad matchups. We've seen ZSS vs. spacies and Roy a dozen times now, and ESAM vs. Maister frequently involves at least a couple of Pikachu games.
Or, the most prominent example near Leo's skill level: how many times has Tweek rammed his face hard into a bad zoner matchup for Diddy (Pacman, Olimar, etc) without switching to Wolf?
Something to keep in mind that's influencing our opinions of both Byleth and Aegis (but more so Byleth).