I never played any of the DOS games so I don’t know how EA went with those games. Was there serious balance issues in DOS at roughly the same development cycle as BG3? Were they ever fixed?
I played DOS 1 and 2 including EA. Small bugs were fixed, but I don't recall any massive changes based on players who disagreed with certain aspects of the design. Larian tends to stick to their major design decisions. Either that, or by the time players are able to give feedback in EA, it's too late for the changes players would like to see.
For example, DOS1 had an issue where melee-based characters in your party were not as effective compared to magic users. If you didn't choose a party with a heavy tilt towards magic you'd have a very difficult time. There were also complaints about the more "goofy" aspects of the story line with attempts at humor. They didn't change any of that in DOS1 but they did tone that down a bit in DOS2.
In DOS2 there was a brand new armor system that some players absolutely hated (I didn't love it myself, but never felt that strongly about it). They never changed that, as far as I know.
The overall impression I have from those games and how they're handling this one, is that they're very interested in fixing bugs, and they may slightly tone down something here and there where the fix is easy, like eliminating a few of the surface effects resulting from spellcasting. But they probably won't change any fundamental aspects of the design like the party size, the camp rest mechanic, or the use of homebrew departures from 5e rules. Either because it's what they want, or it's already been baked into the cake and it's too late to change.