9. Live Service Shooters
Though balancing most online games (especially ones with varied and contrasting ability and equipment pools) is already a tricky challenge, the distinction of "most difficult" in this category goes to the live service shooter.
Unlike a session shooter like Counter Strike, a live service game has to be marketed, monetised, and balanced as if it's an organic creature - always on, always awake, always available.
This requires a deceptively high level of resource and overall effort... because whilst it might be tempting to just maintain the level of success and engagement your game currently has, the interest will wane rapidly without regular and significant content updates. And be assured, that if your designers estimate the content your team creates will last the average player twenty hours, that just means one maniac is going to blast through it in five, then complain there's nothing to do.
So, it's worthwhile to have two development strike teams working in tandem - one to balance and maintain the game as it exists, live, and another to produce the content that players will voraciously guzzle down in the future.
After the latter team deploys that content, it'll be the former team's job to balance that new content, and so the cycle continues.
"Here's the new battle pass we've been working on for months! It's four-thousand levels of difficult and exciting new content, ready fo... aaaaaand, someone's completed it already.