Let’s say that we have a game called Boar Quest and we have two testers. Tester A is hired by company and tester B is hired by a third party. The goal is to get a Shiny Rock by killing Fluffy Boar which has a 1% drop rate.
Tester A’s job is to kill Fluffy Boar in the real game (so that means if it is a random encounter going through that random encounter system) and get some number of Shiny Rock to prove that Shiny Rock drops from Fluffy Boar and to show that the rate is as expected. There are some algorithms in play here but overall real play testing requires real gameplay so even at 2x speed maybe it’s a slog.
Tester B is hired by Randy McTwitchknob and their job is to go into the game and combine items and skills and everything else to bug the game out and make it so that Fluffy Boar has a drop rate as high as possible for the Shiny Rock. They find an interaction between two of the three hundred items in the game (which are supposed to be unrelated and work as intended independently) and a skill which makes the drop rate 100%. Randy takes this information, pays tester B, and makes videos showing the exploit.
Tester A’s job was to make certain that the Fluffy Boar dropped the Shiny Rock, to ensure that the items in question for the exploit worked as intended, and to ensure that things functioned so that the game could be played. This was successfully done. Tester B’s job was to undermine the code, find exploits, and profit from them by selling the knowledge to Randy and this was also successfully done.
Exiting the example above this has plagued games since games came out though before it was GameFaqs and some guys in their underwear chortling together that they found an exploit but now it’s a serious business worth quite a lot to break games. So it is not that class balance is hard because tester A is just slacking on the job but because tester B’s whole job is to break that balance. There are many games where, without exploits, the game is quite fair but with exploits, secrets, or knowledge that shouldn’t be yours the game breaks and because people literally get paid real money to sit and break games the moment they come out there’s no way to really have a public facing balanced game anymore.
So why is this class better than that class almost immediately on release?
Because some guy spent twenty hours day one for a few hundred or thousand dollars breaking it. This is why the PTR / EA / feedback process is so important and why bug testing got outsourced in a sense. I mean, certainly part of it is financial, but the majority of it is comes from the fact that tester B’s mere existence renders tester A’s range to be greatly diminished; their job is to make certain it works and doesn’t crash all the time for in-game reasons if possible and that’s where it should end given the modern era.
Just to be clear POE2 is going to have guides on how to play it in less than a week and lists of best builds and class exploits. Mark my words these people exist.