I find it more baffling that this unfairness is acceptable in a multiplayer game.
Tanks are unbalanced.
The modifications of tanks makes it unbalanced by letting some tanks unlock a whole extra ability (artillery strikes).
The RNG of shooting tanks is unbalanced and just makes everything feel like a dice throw when playing.
And this has tournaments, the kind where they determine "who is the best?"!
I love the underlying gameplay, driving tanks is fun, the potential teamplay that could exist is fun, but this is just too randomized and annoying. Sometimes I can repair my tank, sometimes I can't. Sometimes I get to get into combat, sometimes I am one-shot out of nowhere because they hit me square in the ammo dump while I was moving, from a kilometer away. I got one-shoted by a airplane out of nowhere that hit me in the fuel dumps...that was fun....
You are right, that the modules that you research and enable should increase the BR of the tank. But of course it would add an extra element of complexity to the whole BR balancing thing, and it has plenty of complexity already. Armored Warfare has this, but of course it's a much simpler arcade game with health bars instead of (semi) realistic damage models.
In War Thunder your crew basically has experience points. If you increase their vitality, they stay alive better when taking a hit and recover faster from unconsciousness. And of course boosting repair and reloading skills help a lot. Some high level players seem to enjoy playing with low tier tanks for one reason or other, so if you are used to playing with newbies, suddenly meeting a level 60 guy with almost maxed out übermen crew may seem pretty unfair. But that's part of the whole massive online multiplayer and free to play experience... you can also pay money to boost your crew skills if you want to save time. But just because of this, the playfield is never even. Some people have advantage. And people with fast PCs always have advantage over PS4 (or low end PC) players, and people playing with mouse controls have massive advantage in airplane side.
Medium and light tanks get artillery, heavies and tank destroyers don't. RNG by nature is random, not unbalanced. I think it adds realism to the game, there was big differences in ammo quality, and steel quality etc. in real life, and this can be simulated by adding an random element. It works relatively well, if not perfectly. Previously a hit to the ammo rack almost always destroyed your tank, I think that was more unrealistic than the present randomness. Previously some people had unnatural skill in hitting you right to the ammo rack from the other end of the map... Of course the guns are still unnaturally accurate, no one would try to hit your gun barrel in real life, for example, but in this game it is done regularly.
I think, as a PS4 player, the biggest problem is the PC players ability to cheat by disabling grass and bushes, and seeing your tank when you think you are well hidden. This is of course mainly problem in realistic and simulator modes, which is why I prefer arcade.
Another is how some people abuse the "spawn as a fighter" in arcade. When you enter an air event, they look which kind of ground device you are using, and if it can be killed with a fighter. If yes, then they jump into a fighter, and fly straight at your vehicle and destroy it, while you are defenceless. And of course some people just jump into a bomber or assault plane when they get killed, and try to revenge their deaths by bombing their killer. Removal of name tags during these air events would help so much.
And of course meeting tanks with +1 BR can be annoying, if you start seeing it too often. I'm playing American line with BR 5.0 lineup at the moment, and far too often I''m seeing Tigers (5.7 or 6.0) in the opposing side. It would be okay if it was just 50% of the time, but it feels like there's too many players playing with 6.7 and 5.7 lineups, so the matchmaker throws you in with them. Of course you can make this work for you too, if you play 6.7 lineup yourself, then you are not likely to meet many >BR7.0 opponents. BR 6.7 is a touch place though, sometimes I prefer playing more with my BR 3.3 to 3.7 T-34 lineup. And BR 7.0 is a hellhole, you are meeting BR8.0 tanks all the time, and you are pretty helpless against good Maus and IS-4M players.