In my opinion the abuse of furtuitous coincidences in a story is a lot worse than the overuse of a particular device, especially when the coincidences cannot be explained if not by a precise design of the author, and therefore it's a sort of deus ex machina. This is even worse when you're dealing with a "mystery" where you're supposed to use reasoning and logic, and therefore by logic you shouldn't consider plausible that a series of events that clealry follow a design are just coincidences.
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