> It seems like C++ libraries are getting more and more configurable in a way that hurts readability and understandability and library designers don't seem to recognize there is a fundamental tradeoff here.

well, I mean... if it was not configurable, it would not really be useful, except for toy examples, and for those the default std::unordered_map is more than sufficient. To be useful, a hash table implementation at least needs to allow to configure the hash function and the comparison operator.