I can't agree with the conclusion. Sure complexity is complex, but the solution here would seem to be better performance analysis of the standard library rather than blindly avoiding risk. How do you know where to stop? All built in block-accepting methods for enumerator types would seem to be equally risky in this analysis.
Without much knowledge at all, I'd agree with the conclusion that "block-accepting methods for enumerator types would seem to be equally risky". It sounds to me like having non-pure functions that can terminate at any point. And that sounds like asking for trouble in your code.
