I'll be very surprised if I ever find a story that's flawless except for insurmountable mechanical problems; usually people who are that good are intelligent enough to know that they need a beta, particularly if (possibly the most likely case) they are non-native speakers. It's also a much smaller problem, even if the mechanical errors are such as to require significant revision, because it's a problem that is solved simply by... finding a beta, so not as poignantly heartbreaking as when the writing is flawed and prevents a story from being good or great.