I consider e.g. 'scope' to just be (slightly harder to test and follow) over a class method.

I consider a concerns to be more complex and unexpected versions of a Module.

I consider "render @foo" to be just an incomprehensible version of the explicit "render_partial("the_name", ...)" And so on.

All of these have bitten me many times, oft in production even. Many of these caused me and my step debugger hours of wading though deep stacks of method-missing or runtime code definitions.