Comment by darkwi11ow

It is combinatorial explosion of possible states (not as in memory state of program, but the differing visions of the next state of architecture). Plus high impact of mistakes, suboptimal solutions and randomness when switching between states.

Edit: I like this 10 years old blog post in regards to explaining the problem much better than I could do: https://pron.github.io/posts/correctness-and-complexity