Netcode
The netplay was probably the biggest and most important unknown going into SF6's beta, since it would make or break their game. My internet is also notoriously awful, often connecting at high ping with lots of packet loss, so it was a good use case for the netcode.
After a bad first game (which could be from any number of factors), I played more matches against friends in specific locations, and I gotta give the netcode high marks. You can see the rollback working in high ping and I never had any problems with desyncing, and matches remained playable beyond the 150ms mark, even with high jitter. If your internet isn't as unstable as mine and you'll be connecting at 100ms ping or less most of the time, I don't think you'll have any problems. Certainly, playing anywhere in the US/Canada vs. each other should almost never cause problems, and west coast to Japan or east coast to Europe should be plenty playable most of the time.
The one main feedback I'd like to give is to have the option to set your input delay to a fixed amount, rather than letting it fluctuate as the ping gets higher. The rollback model works perfectly well even if different players are operating on different input delay, and having input delay change in the middle of a match is not very ideal, even if it happens relatively rarely. It's fine if the current fluctuating delay method remains as the default option (perhaps called "Automatic" or something), as long as we can also set it to 0F, 1F or 2F of fixed delay that never changes.
That said, the netcode in its current form is vastly superior to any other Capcom game and should hold up to the standard set by Guilty Gear Strive for rollback implementations by a Japanese developer. I'm kind of handwaving this as if it's not a big deal, because this post is long enough already, but really, it's a nice relief that this game will be playable online. With an option for fixed input delay, we should have no problems going forward.
Battle Hub & Lobbies
I'm a person who generally just likes to press play and be matched up without a ton of bells and whistles, so I didn't really know what to expect from the battle hub. But after spending some time in the hub, I quite like it.
The camaraderie of the arcade setting is a warm feeling, and it's fun to run around and see people's goofy avatars and emote with spinning bird kicks. They've taken care to make the lobby pretty navigable, since you can pause and warp to any player or any location at any time, see an overhead map to find an open cabinet without having to run around, etc.
I didn't really like the fact that you can't control how long sets are on the cabinet; if there's a line, it's always one game before the loser is booted. We don't have access to private lobbies or know how they work, but I think it's extremely important for Capcom to allow massive control over how sets can be played in at least one of the online modes. If they want us to hang out in this virtual space like we would a real-life local gathering, we have to be able to control which players are playing like real life as well.
We need to be able to control how many games are in a set before the players cycle out (FT1, 2, 3, etc), and who gets off the cabinet (winner stays on, a specific player always stays on whether win or lose, a.k.a. "streamer mode", etc). If I'm playing a long set with my friend, hopefully it is also easier to switch characters than it is now (requiring us to stand up from the cabinet, open the profile and switch, then sit back down). Can "switch character" be an option on the online rematch screen and simply pop up the mini-character portrait menu right there?
Ideally, as a far-reaching ask, it would also be nice if we could queue up in line on one cabinet while still doing other things, like playing in other (unranked) matches against other people. If our place in line comes up, a message appears over our current match and we have 15 seconds to accept, or else our spot in line is lost. We would then immediately forfeit the current game (not a big deal if it's unranked) and go play the other match. I think this would be a pretty common use case especially for streamers who have long lines of viewers who want to play, without having to manage the line with tools outside the game.
I also used the spectator mode a lot, and I love it. It's a lot of fun to go around to each arcade cabinet and just watch matches, complete with live inputs and frame data (!!), and also giving us the ability to fast-forward to the current live spot in the match or just watch from the beginning (Capcom clearly added this feature with esports viewing in mind).
A small criticism of the spectator feature is that you are kicked out to the lobby every time the next person in line steps up to play, and you have to re-select the spectate option. I would love it if I could just watch the cycle of matches indefinitely without having to constantly rejoin as a spectator. If just two people are playing and rematching over and over, then you do just watch all the matches back to back with no further inputs needed, and that is excellent, so it's only a problem when people are cycling in and out.
Also, when spectating, allow us to see the total set score and win streaks. This will really help us broadcast sets, as a spectator, between two friends playing. I think this feature was supposed to work, but it was buggy in the beta (the win streak would never change from the first value when you started spectating, even if that player lost). Even if you have to only show the set score on the victory screen, that would be okay.
Also, the game constantly changes the spectator's music and stage randomly between each match, which is a great decision that really helps variety. The fact that all players can be playing or watching the same match on a different stage with no detriment to the gameplay is fantastic.
Tournaments are a cool idea, but they seemed buggy in the beta. It didn't feel like all the first round matches were played concurrently, and instead we had to wait for each individual match to be completed before the next one could start? This led to tournaments taking forever. Also, it didn't seem possible to spectate the tournament matches, or do anything else (like shop for avatar clothes or practice in training mode) while waiting for your match. Maybe this behavior will be ironed out in the final game, but it needs a bit of work.
I'm sure it's on Capcom's mind already, but I'd love to see seasonal decorations in the battle hub. Christmas or Halloween decorations at the appropriate time of year would really liven the place up, and make it feel like an actual physical space. Capcom could even promote other properties like Monster Hunter with decorations when needed.