The big difference between the QT Atom structure and RIFF is that RIFF is a series of independent chunks (IIRC), whereas Atoms are a big tree. Structurally nearly identical, though.
Don't know about RIFF, but IFF files are/can be a big tree - the outer chunk must be one of FORM, LIST or CAT, and many chunk types contain additional chunks, so depending on the file you might get structures of arbitrary depth.
