Could you be more specific? What makes it painful?

(I mean, "overly complicated" is bad news no matter what the specifics of what we're talking about, but how does it cause pain when it's specifically in the types?)

For me personally it's just a lot of really heavy and very "meta" code that is really hard for me to reason about. I also touch it very rarely, so there's always a steep learning curve when I'm jumping back into it.