Those concerns voiced with 2.0's refinements before release were why they were marked as experimental, and were the driving force behind the changes made to refinements between 2.0 and 2.1.

On the 2.1, non-experimental version, he has said: "What we’re going to have as refinements in 2.1 is a paired down version that I think still provides the bulk of the functionality people wanted out of the feature without most of the pain that I was concerned about originally."

http://rubyrogues.com/124-rr-jruby-with-charlie-nutter/

2.1 changes how the method cache works. It is now hierarchical as opposed to the old global cache.