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.
Let me go through your points one by one.
As I've said, I'm not against homebrew as a concept. You're very right that the majority of players won't care-I certainly didn't. I did not like the fight, but not because I don't like that they changed the phase spider's stats. However I don't really see what diversity is actually added by changing the spider. Plus what I've seen of how a phase spider is meant to act sounds interesting as well. Sneak-attacking at random could have created a lot of tension in the combat. It's not like there are any enemies that could already do that in the game.
I too think that homebrew can make the game better if done right. I'm just not convinced that Larian is doing it right. I don't mind surfaces in principle-though people have pointed out elsewhere the ways that it disrupts other D&D systems-and I even enjoyed an early use where using ray of frost on burning sections of ground could put out the fires. I felt really clever figuring that out. I'm not ever going to advocate never changing or adapting things from one media to another, but I think that sometimes the best improvement that can be suggested is "this homebrew attempt isn't working, it needs to go." Sticking to the surface example, I'm ultimately fully neutral on those. If they stick around in an improved form, I'll be fine with that. If they get removed completely, (which I don't expect to happen) I'll be fine with that too. Surfaces are a thing that I don't honestly think will enrich the game meaningfully even in their best version. Nor do I think their presence will negatively impact the game once they've been altered and improved.
Firstly let me say that I'm never going to advocate for perfect world consistency at the expense of fun or interesting story. Quite the opposite, I think story or enjoyment should always trump strict adherence to logic. But there's still got to be an internal logic to the world. Saying it's a fictional universe doesn't absolve it of a need to make internal sense. Saying "it's a fictional world, it doesn't matter" isn't a good rational. There has to be some line of logic that players can get invested in. I'm not saying the poison thing is a cardinal sin, but they don't even try to justify it. With your arrow example, you can hit someone in steel armor with an arrow because the arrow hits one of the chinks in the armor. Ray of Sickening is explicitly magic, it's not hard for an audience to justify that working in their heads.
In my opinion the poison thing really isn't a big deal, but I'm against saying, "it's a fictional world, it doesn't matter." I think the bar should be, "it makes things more fun/interesting/easy to use, it doesn't matter." Dismissing a hole in logic just because it's a ficional world absolves the creators of too much responsibility.