It's complicated !
Complications arise when the evaluation of the position becomes uncertain. The position becomes uncertain because the tactics become so complicated that it becomes difficult to analyse without missing something.
There are several ways to get there.
- Piece sacrifices. When you sacrifice a piece, you do so in order to get something in return: An open enemy king, piece activity etc. Valuing all these things against each other becomes difficult, and you don’t know which side is better.
- An open center. When the center is open, there are often far more tactical opportunities.
Diagram - Black to play moves 42... d3 where his pawn is attacked 3 times but only defended twice. When White captures with the knight, what will Black play ?