I've been pretty happy with Samsung's update speed of the US Unlocked model so far. I remember it started pretty bad the lag between the US carriers getting updates and the unlocked model S10+ I had previously. But got a lot better over time. Now at worst comparing the release dates here on Samsung's site
https://doc.samsungmobile.com/SM-N986U1/XAA/doc.html
to AT&T's software update history page for the Note 20 ultra at worst it seems the unlocked model might get a update about 2 weeks later.
I'm more than fine with that in exchange for having the cleaner (software wise) unlocked model. And not having certain features completely disabled just because the carrier wants them disabled because they sell a app with a premium sub. Features like the built in caller ID and spam/robocall detection and blocking. Also thrilled that finally Wi-Fi calling works on US unlocked models while using AT&T since the Android 11 update (even my U1 S10+ got it)