Around 2005 I got heavily into expression templates and various compile-time tricks for making libraries that were very expressive to use, like internal domain-specific languages embedded in C++. In particular a fairly complete embedded SQL thing, similar to what has since come out as Linq on .NET.
For users, it's fine. But for anyone else apart from me trying to maintain it, it presented a massively steep learning curve. So that's the problem with it; like any "clever" technique, it reduces the pool of people who can maintain it.
This is fine for widely-used class libraries, which general users never need to understand the guts of. But for "in house" libraries owned by a specific team, they probably all need to be able to patch it or extend it sensibly. The more arcane possibilities of C++ templates seem to preclude this, in my experience.
It was a lot of fun though.
