D20 system at face value is fine. It simply is a way to quantify a randomness and relative potency of modifiers.
The issue that can cause aggrivation tends to be the "Natural criticals" aspect of D&D. Where, irregardless of DC and modifiers and Nat 1 is always guaranteed to be a miss/fail and a Nat 20 is always guaranteed to be a crit/success. Which means that no matter what you do, you're always subject to a 5% chance to get screwed.
Which can be frustrating. Especially in extreme cases like you're rolling on a 5 DC skill check and have +16-20 modifier that should absolutely definitely guarantee you are way more capable than necessary and you still fail. Or an enemy that has -10 Str and a 1-4 penalty on attack rolls hitting your 25 AC character can still annihilate them with a critical hit.
Which, while it could be worse (There are TT iterations where for you to crit you needed to roll to confirm and critical misses could not only cause you to fail an action but also make it backfire like instead of shooting an enemy with your crossbow you hit your ally in the back of the head and kill them) it can still present a lot of frustrations, especially since it works to make you feel like your characters abilities are worthless when you build them to be super awesome at this one thing and they still fail anyway.
Of course, there inclusion is necessary, since it's a core part of D&D and since the game is based on D&D it has to include their rules. Larian has mentioned that there are some things that have felt restrictive to them in terms of game design because of being a D&D game and it's possible that the D20 system and its "Natural criticals" are one such thing.
So it'll be interesting to see how Larian's next game turns out, especially if its a new title not attached to D&D (Either Divinity 3 or a new IP) and see what aspects, if any, they bring from D&D to that game.
