I'd say it has less to do with oil (which is a handy tool, but nowhere near as overpowered as it was in D:OS 1 and 2) and more with smokepowder. That stuff can wipe out a whole encounter if placed right, and one has to be really commited to their roleplaying to not just squirrel every SP barrel away to use for clearing out the goblin camp or some of the Underdark fights. It's not quite in the Deathfog territory (which at least had the sense to not appear in barrels until Act 4, but nothing stopped a particularly persistent player from terrain-swapping it into desired places where it was available), but it's definitely up there.
It's also curious whether or not barrels and other containers can be fit into bags of holding if they are to appear in the game. Because that will just remove weight as an obstacle to barrelmancy entirely.
. If the liquid were only as dense as water at 8.3 lbs/gallon, then a single barrel alone would be ~400 lbs. A bag of holding can only hold 500 lbs worth of items, so you'd only be able to get a single barrel in a bag of holding if Larian implemented even basic equipment rules. The reality is that none of the starting characters should be able to toss a barrel around without a bag of holding if carrying capacity is 15 x Strength score.
It's just another example of Larian failing to actually read the rules because barrelmancy was already built into the engine from DOS.
