Just by playing the game normally you can find out about how strong this stuff is
Oh i do play the game "normally" ...
I have 411hours played ... since start i finished Early acess more than 20 now ...
And i tryed it all ...
I tryed eating food during the combat instead of drinking potions, i find every combat trivial and unenjoyabe ... i stopped ... my game experience improved.
I tryed to spam Long rest after every combat, i find it tedious and anoying ... i stopped ... my game experience improved.
I tryed to hide after every attack, i find it simply stupid (and also i was forgoting often

) ... i stopped ... again, it was better.
I tryed to walk around enemies to get backstab advantage ... same story as with hiding.
I tryed to kill enemies by throwing them to another enemies ... that was fun, for a while ... but eventualy i stoped again.

I tryed to kill every single NPC i met before even talking to it ... that was interesting experience ... some hard moments was there, if you dont have resuplying vendors that are still offering another, and another healing potions, but you have to do with what you get ... and you have no party with you ... i admit that i never managed to kill Githyanki patrol like this. :P
It's interesting and confusing to me that you're saying not using those things improved your experience, which is exactly the point.
Many people also see part of the game as a challenge to overcome, using the rules of the game. By having these cheese tactics in place it removes that challenge.
You see removing/toning down these features as ruining someone else's fun, but having it in there actually affects people's fun too. Building a barbarian that does tons of damage doesn't feel so badass anymore when in that same universe anyone can just blow each other up with tons of free explosives everywhere.
Now add to that the amount of explosive barrels available (which is also immersion breaking)
This is what i dont like about all those "immersion breaking" people are talking around here ... you just state it, and concider it a fact.
How? Why? ... What is so immersion beaking about the fact that flamable materials exists, and people are hoarding them?

What "immersion" you have, so its breaking for you with the fact, that people are creating wine that is flamable?
(I wonder how they do it, tho since as far as i know, lowest amount of alcohol to make fluid flamable is 30% ... and that can certainly no longer being called a wine

)
What "immersion" you have, so its breaking for you with the fact, that people noticed that oil is flamable, and they started to gather it for that reason?

What "immersion" you have, so its breaking for you with the fact, that people noticed how incredibly usefull black powder is, so they started to create as much as they could? O_o
I mean except the word "wine" wich could easily be replaced by "liquer" (even tho port wine have quite high amount of alcohol, yet its still a wine ... not sure if that is flamable tho) and sudently, all is fixed ...
That all is just natural course of things ... you can see exactly the same was happening in our world history.

You explain why it is so yourself partially. It is a fantasy world so we'll say it's fine they make that stuff.
If there was so much explosive material around and everyone has magic pockets, there would be little need for knights with swords or even wizards that can conjure up a fireball. You could just bring explosive barrels, roll them at the enemy and throw a firebolt cantrip. It makes all classes unnecessary and the world less believable.
Something similar to oil should be rare and expensive. This is more like gunpowder, which should be even more rare. Believability tends to be stretched in fantasy, but there is a limit to where it starts to damage immersion.
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.
I believe you can be pretty sure about this ...
Question: Can you overcome any encounter without blowing even a single Barrel?
Answers: Yes > therefore game is not ballanced around it. ... No > therefore game is ballanced around it.
That was not so hard, was it?

Yes, Barrels are covenient "easy button" prepared for you whenever you might need it, or simply want it ... and that is exactly what they are, nothing more, nothing less.
Are you still sure about this when the highest difficulty comes around and you can't get past a certain encounter that has explosive barrels right next to it? Is someone who has trouble on their first playthrough sure about this? You have 411 hours into this game, it's easy for you to know the intention now, but someone who plays it for the first time doesn't know this. Having spent time researching and creating your first very cool character, only to find out that everyone with explosive barrels is more effective is a very disappointing experience.
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.
I agree ...
But it would be tedious and boring to drag the barell across whole map, just to use it somewhere else, than DM expected you to want it.
Therefore there is many of them, so you can use them anywhere, and any time you want ...
And if you dont? Well, then you simply dont.

No harm made.
Having to put in effort makes it more rewarding, which = more fun.
I don't know if you play D&D, but finding a barrel of explosives can be really cool. Then figuring out a way to bring it along can be really cool when you blow up a camp of orcs or something. Now imagine that every encounter the DM describes what you see and then says there is an explosive barrel in the middle of it. You stop exploding them because the third time it's already stopped being fun, but the DM keeps telling you about them. You'd start to roll your eyes a lot. That's what it currently feels like.
I believe Larian did fantastic job here ...
They realized that this game will be accesible for wide amount of players, with wide amount of expectations, and even wide amount of definitions of fun ... then, they implemented as many options for those people as they were even able, so everyone can have his own.

Sadly ... then puritans came ... and start demanding that litteraly everything that is not in the book is scraped, destroyed, remowed, thrown away, and forgotten for eternity ... what we get in the end, if Larian will listen to them and only them, would be game that would be just perfect for the most hardcore DnD players ... quite good fot casual DnD players, or experienced RPG players ... and will totally suck for everyone else. :-/
I dont want this to be BG-3 fate. :-/
This has nothing to do with puritans, you can have explosives in 5e. It's about people who want to actually have their choices matter and the world to be believable.
but they want to feel clever for using it and it should be unique to a certain experience.
And nothing is stoping them, but themselves.
That is another funny part.

Nope, the option is shoved in your face so much that it doesn't feel clever anymore. It feels like it's what the designers wanted you to do.
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."
But those options are not exclusive to each other ...
When you are able to do both ... the one who wants to do first thing, can have his fun ... the one who wants to do the second thing, can have his fun ...
When you are able to do only first ... the second gets screwed. :-/
When you can do the second, the first option doesn't feel good/clever anymore. Challenge/effort makes things rewarding. Additionally, an explosive barrel that you brought from town to the other kind of the map also doesn't feel like it's what the designers meant for you to do, but rather something you came up with and is specific to your gaming experience.
Your whole problem with not understanding this is in what it feels like to do these things for most people. You feel it's fun to just try different things. Other people think it's fun to come up with clever strategies and outsmart the game, or to be challenged. Being "clever" in this game is too easy, because everything is just allowed. Constraints breed creativity. Someone who came up with really cool party tactics can actually be disappointed when they find out that barrelmancy (or shoving everything for that matter) is just better. It's fine if it's there, but it needs to be balanced around other choices.