Every time I deal with ifdef mazes I regret it. MonoGame is a familiar & nasty example, since it tries to be a portable reimplementation of a game framework (with ~a dozen platform targets/backends) but instead ends up being catastrophically buggy - as an end user or a maintainer you can't tell which functions are even implemented or syntactically valid.

I started using a SDL2-only fork of the same library (just one backend! near-0 ifdef count!) and suddenly all those nasty problems went away.