As has been pointed out before, Larian has a whole host of potential monsters to choose from. Sticking with the phase spider encounter example (which I dislike for reasons unrelated to adaptation stuff), why make the creatures phase spiders if you're also gonna give them a bunch of abilities that are already present in another spider monster?
As I mentioned in another post, they're just making things harder on themselves without much meaningful gain. Like, what is gained substantively by changing the stats of monsters when there are other monsters that could fill the same role without changing their stats?
5e is light on Raw spiders types as far as i know, nothing wrong with homebrewing a few. There is a well known sword spider in game already and those are from 3.5 if i remember correctly. What we get is diversity and millions of casual players will not care one bit what stats were used.
Regarding homebrew in principle, yes I agree that homebrew is in the spirit of D&D and TTRPGs as a whole, but this isn't just someone's personal game around a table. It's an adaptation of a beloved setting that is also the sequel to a beloved franchise. I don't think it's unreasonable, with that context, to expect that Larian would take an attitude of "stick with the setting as given unless changing things gives a clearly better experience." A lot of the changes I've seen discussed seem to just be changes for the sake of changes. And when taken into consideration with a lot ofthe other system changes that Latian have made, it's not unreasonable to think that Larian doesn't actually care much for the property they're adapting. Honestly, speaking as someone who themselves doesn't care or know much about D&D and knows literally nothing about the Forgotten Realms as a setting, Larian is approaching this game in a way that makes me think they actually care about D&D either. Not that I think they actively dislike it, it just seems apparent that they're more interested in doing their own thing and bending the D&D acoutrements to fit, rather than making an effort to exemplefy the benefits and strengths of D&D's system and setting. Their approach to monsters is just a microcosm of that.
it's not unreasonable, but you are forgetting that millions of people like their "thing" and Wotc thinks Larian way is the way to go. They are the gatekepers of DnD. It was clear from day one at least to me that surfaces will still play a part in BG 3. THey did made it more subtle.
i think there is room for improvement. But throwing homebrew and adaptation under the bus because one hates Larian surfaces or whatever just isn't my cup of tea so i can't support that. I believe homebrew will make for a better game if they do it right.
What i do support is improvement and suggestions that add or change those adaptations.
Also just to touch on the poison issue, the fact that poison seems to seep into your boots to affect you *is* kind of silly. Not stritctly world-breaking but another example of them deviating from how D&D works for seemingly no reason beyond that's how they wanted to do things.
Well, if you go down that road it's silly that arrows get through steel armor as does sickening ray, we shoot made up arrows and other stuff as well. It's fictional universe as i said before.
