Bitrates lower than 32 kbit/s (below "a" preset)
With the presets, it is not possible to reach very low bitrates such as 12 kbit/s or 16 kbit/s although they are supported by e.g. Mainconcept's web encoder: https://www.mainconcept.com/cclab/xhe-aac-encoder
It would be cool to have a preset that produces smaller files :)