It's used to mean 'bad things will happen to you', so 'woe unto anyone who criticises this'. ' But I haven't heard it used with an adjective as you do above - you could do it, but it would be unusual. The phrase itself is unusual - the word 'unto' is old-fashioned and now seen as 'poetic' so the phrase is usually used jocularly (as a bit of a joke) and I'd be careful about using it.
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