Hmm as far as I know, Samsung is the only android oem(apart from Google) that provides monthly security updates on time. Oneplus does this bi-monthly and in the past the update schedule was even worse. As for Mi, let's not talk about updates. They barely do any QA , examples being the mi a3 which literally got bricked after the Android 11 update and the poco x3 whose camera stopped working after an update. And there are numerous people complaining about buggy miui.
As for one ui 4, s21 series will get it by December. They have to skin the vanilla android and then do thorough testing so as to make sure they are no major bugs.
One UI 4 beta for s21series had already started in October. But beta program became full by oct end. So, Samsung is not allowing anyone to enroll. You will directly get stable version by end of the year.
