Why do green threads always get thrown under the bus unequivocally? Aren't Ruby's green threads essentially coroutines that yield the processor when they wait f

Why do green threads always get thrown under the bus unequivocally? Aren't Ruby's green threads essentially coroutines that yield the processor when they wait for IO? So for an IO-bound app, which seems to be what evented framework users are concerned about, why can't you just write normal synchronous IO calls and run multithreaded Rails on C Ruby and be fine?