I found their ELF format specification to have a decent coverage, even if not completely exhaustive (e.g. some debug info isn't breakdown after a certain point, but it just might be incomplete rather than limitations).

Things may be non-byte-aligned bitstreams.

* https://doc.kaitai.io/user_guide.html#_bit_sized_integers

Arrays of structures that go "read until id is 5, but if id is 5, nothing else of the structure is emitted."

* https://doc.kaitai.io/user_guide.html#_repetitions

Fields that may be optional if some parent of the current record has some weird value.

* https://doc.kaitai.io/user_guide.html#do-nothing

Files may be composed of records at arbitrary, random offsets that essentially require seeking to make any sense of it.

* https://doc.kaitai.io/user_guide.html#_relative_positioning

The metadata of your structure may depend on some early parameter (for example, is this field big-endian or little-endian?)

* https://doc.kaitai.io/user_guide.html#param-types

* https://doc.kaitai.io/user_guide.html#switch-advanced