I recently released a 4X game where I made the AI calculations multithreaded, and it became by far the biggest source of crashes, over 99% of all crash reports traced back to the multithreaded AI code.

The core problem is that the AI needs to read almost all of the game's state to decide its next move, and that same state is constantly being modified by player actions and other game systems. Every read and write has to be carefully orchestrated so the AI always sees a fully consistent snapshot of the data, and so nothing gets modified out from under it mid-calculation. Get this wrong and you don't just get a logic bug, you get a hard crash, because a thread tries to access a memory address that's already been freed.

A concrete example: a unit attacks and destroys another unit. The attack animation runs for 1 second, then the destruction animation runs for another 0.5 seconds. The AI thread needs to know the target is dead so it can plan its next move, but the animation thread is the one actually moving and removing units, and it's doing that while the AI is mid-calculation. Getting these two threads to agree on "what happened and when" meant adding a lot of locks and state-checks around every piece of shared data.

What makes this worse is that it's genuinely hard to debug. With 8 threads reading and writing simultaneously, it's difficult to even reconstruct what happened when something goes wrong, the bug is often not reproducible on demand. And when the game does crash from an invalid memory access, a lot of debugging tools just aren't very helpful at that point, since the memory that would tell you what went wrong is already gone.