One thing in the beginning a single player game does not have to be perfectly balanced. As we have already explained it, let's move on. Some changes from the DnD rules are needed, otherwise the fight at low levels would be quite limited. Due to the fact that if a player becomes discouraged at the beginning of the game, he will most likely just return the game.
Because of this, we have changes in actions. Changing the shove or disengage is intended to give the player more things to do on their turn. Let's skip the discussion of whether it's a good or bad change, but it does what it was supposed to do. I guess they could go fix it a bit by adding a disengage (even as a bonus action)
Next we have the surfaces. Personally, I think it's a nice addition to the game, although it needs some changes. First, the fire must be weakened. Entering the fire should deal damage only once per turn (roll reducing damage by half can be added), or it should only set the target on fire, but in this case the damage should appear only from the next turn. I don't think other surfaces are a problem.
Cantrip with surfaces shouldn't be a problem. A fire bolt with reduced fire surface effect shouldn't be a problem, but I think ray of frost shouldn't create ice at the target unless it is standing in liquid.
I don't see a problem with the barrels, I don't remember a fight where it would be a problem (maybe in the zhentarim hideout, but it made sense in terms of the story). The game does not require you to use them at any time, they rarely are positioned in such a way as to be able to blow up enemies without placing them first. And the fact that you can do it is not a problem. If you want to spoil the game then do it, having a lot of freedom in how you want to play is great, at least as long as the game doesn't force you.
Now we come to the advantage system. Here I have to admit it is broken. Still, it shouldn't be that hard to fix. I believe that the backstab advantage should remain, but it needs some change. It should only work when one of the allies is standing next to the target. This would add an extra tactical option and encourage proper positioning, but it will only work if they add a disengage as a bonus action, which would make positioning difficult (As you are unable to pass through npc). The distance advantage should be removed and you can add +/- 4 AC instead. The change would also have one additional plus, increasing the incentive to fight in close combat.
Coming back to the fact that players don't like to miss. Larian certainly hadn't made up this information. Otherwise, they would not waste time trying to balance the system. I'm sure they have access to data from previous games on this subject (and they probably collect this information from BG3).