To be fair, I believe balance is something you do for a mechanic, and a mechanic is something you implement into a game. Balance, to be fair however, can be a pain sometimes. Especially with a large cast/enemies/mechanics.
That said, I believe the hardest thing to implement into a game is any form of diverse lighting. Now, keep in mind, this is coming from someone that very much still needs to learn a lot (an amateur), but when I have to adjust lighting or if some character has to hold a lamp or flashlight, I have a hard time.
Additionally, any time I have to do some eventing, and there's a lot of variables involved that changes up said cutscenes. It's still something I have trouble with.
I'm still having a hard time implementing a CCG type function >_< I almost had it in VXA, but then my puter gave up the ghost and took the backups with it. (lesson learned: keep a hard copy of your code!) I hadn't used JS in a decade so I'm rusty AF and banging my head on my desk trying to get it working in MV.
I've been viewing trailers and product banners on Steam as part of studying marketing for videogames, specifically JRPGs. There's a huge gap between the top and bottom in terms of quality and even a mixed review game can have a much better look than a game with great reviews. I can see why skilled publishers become attractive (for hire) to many aspiring game devs.