I came from a background of having very little knowledge on D&D, but know a lot about other tabletop games and have always loved them. Thus when I started playing BG3, I enjoyed googling and looking up the rules of D&D 5e and learning them quite extensively. Quite literally I've spent many hours just learning these rules while playing through BG3 so far. However, were it not for me learning these rules, I'd have absolutely no clue about why or how the game functions. Even in spite of my extensive research, there were many times where I became frustrated with not understanding how certain mechanics worked. I'd say a player like myself is probably in the bottom 10% (or less) in terms of patience and willingness to learn the game.
Early access or not, when a game is released, it's released. The vast majority of players will make up their mind about this game within the next week or so. If I'm being objective, a lack of explanation for the most basic fundamentals behind the core of the game is an extremely poor choice on the dev's part, and one that is rather surprising considering how easy it would be to implement. If players find themselves becoming frustrated this early, it is unlikely they'll bother to come back. This concerns me because I immensely enjoy this game, but potentially foresee poor sales due to a poor initial reception.
Keep in mind that youtubers and twitch streamers have tremendous influence on sales and the overall reception of a game, and so reviews like UEG's are significant. And his review is not the only one that has such a harsh critique on the lack of proper tutorials in the game.
I haven't played D&D since 2nd Edition, & I find the rule set very easy to understand. I have navigated my way through the game without once having to look up the 5e rules. Swords, Maces, Bows etc do precisely what they say on the tin....& you get a list of the modifiers that make up your to hit chance in the bottom left hand of the screen. Spells & Cantrips are also pretty easy to understand. Personally, anyone who has played ANY CRPG shouldn't be struggling with BG3 IMHO.