If you want to be like that, then it was Wormerine who first made an assertion regarding complexity:
cRPGs
tend to be buggy and more complex than your average game, and BG3 is more complex than your average RPG. Frankly, I am impressed how well the game has been polished in spite of its scale.
In any case, when there is a party claiming something and a party denying the claim, the onus is on the party making the claim to provide evidence. Since you cannot provide evidence for the lack of something.
Frankly, I didn’t think it was controversial statement. Going back to any of other titles should make it abundantly clear how less interactive they are. There is of course a question of how much Baldirs Gate3 manages to utilise this complexity, but that’s more of a “was it worth it” question, rather than “what is it”?
So few things I can mention on top of my head: there are very few hard progression gates - per act, you have a lot of freedom as to order you complete the content (and of you complete the content) - and that content has reactivity (so rather than open-world game with small pieces of individual activities that can be completed in any order, but they play out the same regardless, let’s take for an example the infamous Shadowheart that required a lot of custom triggers and hand made narrative content). A lot of companion content happens dynamically, rather than in predefined, linear, set in stone progression gates.
NPCs and objects in BG3 are quite interactive, with individual AI (they will respond to your actions, like sneaking, doing illegal actions, will react to noises, have their own reputation system etc.) NPCs and objects have weight and can be moved around, that has further consequences.
And that all rubs against authored content that tries to accommodate all the chaos that player can do.
Spells and skills have a lot of properties that can be applied and will interact with NPCs and objects in the world. Objects have also permanence - which isn’t uncommon, but it’s worth mentioning due to their interactivity.
Compare it to usually cRPG fare, where systemic interactivity tends to be limited to combat, narrative can be interacted with through rigid scripting of a dialogue system, and the main „systemic” interaction is „attack” NPCs. „You can kill every NPC and still progress” has been the high bar of RPGs so far, and BG3 allows for so, so many player driven changes beyond that. (And that’s ignoring games that straight up don’t allow for unsanctioned interactions).
So really games I think that we could compare BG3 to are:
1) immersive sims - which are more focused, have a lotność systemic interactivity and are generally unoptimised and buggy at launch
2) Bethesda games - that are highly systemic, but compared to BG3 very light on narrative content and are highly buggy
3) Tim Cain games, like Arcanum - which were also quite systemic, with emergent gameplay, and decent amount of narrative content, a lot of player urgency and tended to also be famously buggy.
One needs to reiterate that BG3 does things that those games don’t - like a high amount of cinematics that need to play nice with the systemic stuff.
So yeah, why I personally don’t even like BG3 that much, and I don’t think it’s not as effective in utilising its complexity as let’s say, Arcanum, it boggles me how one could claim that BG3 isn’t an insanely complicated and ambitious title. Permutations and potential interactions are so numerous that testing this game must be a nightmare.