I wouldn't know where to find something out, or I'd get incomplete or wrong information somewhere. Examples -
- the pedia doesn't list each civ's special case buildings, but the wiki doesn't document all the buildings
- in the pedia and wiki vampirism and gift vampirism are listed under different headings with different details included or excluded in each place
- the pedia and 2 places in the wiki say slightly different things about magic immune
Difficulty finding details for whatever reason frustrated me especially because of all the interaction between different game elements.
Seems like you were asking if I think any feature areas are underdocumented. I think magic could use more more precision and more details. I just spent a while with it and think I understand it better, but I had to jump between the wiki and pedia, and rely on some stuff I already knew. Examples of problems -
- the overview pages in the wiki are unclear about how ranks and realms work. It's hard to explain exactly how they're unclear.
- the only place I could find a statement that spell casters get XP automatically is in the magic section of the pedia, which includes an unexplained statement that adepts won't gain experience forever
- the free promotion promotion seems not to be explained anywhere
- the description of some spells is terse (examples - is hideous thoughts permanent? exactly what does mutation do?). And of course, the descriptions are only in the pedia.