One example where it's leaky is when you want to memoize something, and now you need to memoize all its dependencies, recursively, and you end up with a 30 file PR.

I say this as a big fan of React, and I'm hoping the compiler turns out a success.

Well, they're part of the effects abstraction. And they are leaking because you have to manually track them. You are already using the dependency variables inside the hook function. And now you need to duplicate them into an array and keep the array updated as your effect hook changes. This is leaky.