SacAnime uses plastic wristbands as their badges, and they're fairly durable, so they'll last a whole weekend.
I didn't bother doing any age-restricted events when I went to AX last year, so I don't know how their wristband system worked, and I don't usually attend age-restricted events at Fanime so I don't know how clogged the lines can get from ID checking.
But I can say, that as someone who attends SacAnime and has to deal with their wristband system, I don't like this idea, especially if it's pretty much expected that all 18+ attendees will wear a wristband. First and foremost, Fanime is a big con for cosplay, and having a wristband required makes that a lot more difficult for a lot of people, whether it's problems with the wristband being visible in photos or problems with physically wearing the wristband with a costume (especially things like bodysuits and armor). It's also not that effective of a system, since the wristbands are pretty easy to slip off. You can make them tight enough to not slip off, yes, but as someone with sensitive skin and poor circulation, I always request my wristbands at SA to be loose for comfort reasons. It would also make group reg difficult if the wristbands are handed out at registration, since reg wouldn't be able to simply hand someone a handful of loose wristbands and trust that they won't do anything illicit with them. I know that I wouldn't want to be expected to wear a wristband to prove I'm over 18 (probably not 21+ wristbands since this is technically a dry con...) or to advertise to others that I'm over 18 to other attendees.
I think that if something like this were to be implemented, it should be more like the AX system where getting a wristband is voluntary if you want to go to 18+ events. Even then...it seems really easy to just slip off the wristband and hand it to someone else, but then again, I didn't do any of this at AX so I'm not sure how secure it is.