How the software operates is of course dependent on the expressiveness of the configuration format, so it is clearly false in most practical senses to claim that the flat key-value format of INI and BICF will generate the same set of problems as formats that allows for list and nesting.
If we accept the assertion that the complexity of a configuration file for the stated scenario is constant across all configuration formats, we will next be asserting that there's no difference in complexity between solutions in x86 assembly and LISP.
You stated a problem: Configure ~6 servers where they share variables.
I can implement it in plethora of ways. The most sensible one for me is to have a general or globals or defaults area where every server overrides some part of this defaults. The file format has nothing to do with the sectional organization of a configuration file. Because none of the files force you to a distinct section organization.
e.g.: Nesting is just a tool, I don't care about its availability. I don't guarantee that I'll be using if that's available.
I can write equally backwards and esoteric configuration file in any syntax. Their ultimate expressiveness doesn't change at the end of the day.
I don't care. All can do whatever I want and need. Only changes how you parse and map. It's hashmaps, parsing and string matching at the end of the day.
If you know both languages equally well, LISP becomes as complex as x86 assembly and x86 assembly becomes as easy as LISP. Depends on your perspective and priorities.
If you don't know how to use the tool you have at hand, even though it's the simplest possible, you blow your foot off.
Because these are not related to how you write the file, but how your software operates in your mind.