All the usual suspects! Versioning. Floating point. Everyone working on different OS-es. C++'s spec that leaves way too much to the compiler writer’s discretion. Moving targets as C++ and the library dependencies change.
Every dependency just adds that much work. Everyone says each one’s easy, but they add up and eventually we hit the limit where all of our time is spent on maintenance. This is why I’ve been so against adding dependencies beyond Boost and Eigen. Even so, it’s been a nightmare keeping up.