I haven't seen anyone present any real arguments in favor of Larian's design, other than one.

"I like it."

That's about it.
"I like the floor effects because it adds more depth to combat.", when in reality, it strains combat and makes it increasingly more difficult artificially. Not through good AI, or interesting combat mechanics, or varied places where combat takes place.
It's more difficult purely because you need to get your party out of the fire, or grease, or ice ASAP because it provides a massive advantage for such little effort put into it.

"But you can do it too, so why do you dislike it?"
Because then it makes the game too easy, while at the same time actually limiting my gameplay because every fight needs to start with "Ignite/freeze/soak/acidify as much of the floor as possible to make it difficult for the AI.".
This prevents me from making meaningful decisions or taking specific risks in combat like you would on tabletop, because there's essentially only one way to win a fight, and that's by making a disgusting mess of the floor.

As for the action/bonus action economy, it's broken, plain and simple.

And last but not least, Advantage for backstabbing is a terrible idea. It does get annoying always running circles around each other for it, and it's not how tabletop is played at all (nor should it be).
Advantage should only come from Advantage rules, and nothing else.