Providing a unified interface to heterogenous implementations

Question

I am considering the implementation of a caching subsystem that acts as a facade for heterogenous cache implementations.

The immediately obvious disadvantage with this approach is that you lose cache-specific functionality in the abstraction (e.g. System.Web.Caching will have slightly different capability to, say, Memcached). The benefit is that you can let the system decide which cache implementation to use based on its own heuristics.

Is this a bad idea and if so why? What are the pitfalls to watch out for with this approach?

Thanks