In my terminator config I had a [profile] and a [layout] sub-group with name "[[default]]". After reading and writing the config with ruby-parseconfig, only the first entry will be left. The config results corrupted (because of one or more missing parent nodes) and terminator won't be able to load it.
I interpreted 'It also supports nested [group] sections.' as the way some gnome config files have nested groups by multiple '[',']' for every level. But it seems there isn't a hierarchical implementation of groups. Are you planning to add this feature, if so in which way? If not, maybe a small readme update might reduce confusion.