Type slot functions (tp_new, nb_add) have fixed signatures, which makes it impossible to pass extra context. In particular

• you can't easily get an equivalent of Python's __class__ cell, so you can't easily emulate super in heap types.
• we can't pass a HPy-style context.
• we can't change the signature, e.g. when tp_getattro was added, tp_getattr was kept around, complicating inheritance.
• somewhat related, we can't easily implement flags like “this is a legacy function so it needs a GIL held” for nogil.
• (edit) docstrings / type annotations can't be set for type slot wrappers