Damn, some people really like their explosions, huh? I'll try to explain too.
The problem with barrelmancy is, as many other say, that it's an optimal strategy. Every encounter can be easily overcome using it. In addition it's a very easy exploit to find, so much so that it's hard to say if it even is an exploit. Just by playing the game normally you can find out about how strong this stuff is, as it's simply just a feature of the game.
Now add to that the amount of explosive barrels available (which is also immersion breaking) and you've got a situation where you as a player are not sure whether the game is balanced around this or not. If you don't know this for sure and your abilities don't matter for barrelmancy, and thus for encounters, it feels like your choices didn't matter.
The amount of explosive barrels available also causes the problem of removing the novelty of the idea. Bringing along an explosive barrel to win an encounter can be really cool, but when that can just be done every battle, it's pretty boring. The amount of it feels like a DM constantly telling you to use his "fun" solution. A single oil barrel that you could use somewhere, is way more exciting than an oil barrel around every corner.
It's not a glitch, but a feature, which makes it hard to ignore. Larian has a tendency to put in so many exploitable mechanics that aren't glitches, so you start wondering if you're supposed to use them, especially if you're having a hard time in a fight. If it were an obvious glitch, like item-duping in other games for example, people would not nearly have as much a problem with it.
All in all, people (even those who hate it) actually do want to use barrelmancy, but they want to feel clever for using it and it should be unique to a certain experience. This means making it more rare and as they've done add some sort of requirement for strength, because then your character choices matter again. You find a difficult encounter: "Maybe you can stash some loot in order to have the barbarian bring that barrel from the town and then you'll get them!" is way more fun than: "This encounter is hard, I guess I'll take one of those 7 explosive barrels out of my inventory." If it's too easy to do, it simply becomes a thing they won't allow themselves to use or it trivializes everything, rather than a cool option they could use once.
Someone mentioned it's not possible to carry a whole bunch of greatswords and stuff as a counter-argument for immersion breaking. Larian needs to fix their whole inventory system anyway. Choosing what stuff to bring and what to leave behind should be a part of any RPG. It doesn't have to be entirely realistic, but it should matter somewhat, so we're not just mindlessly clicking away. The weight of gold is something people dislike, cus we're used to credit cards and stuff I imagine.
