Not her best mystery, but given that it's one of the earliest Poirots, perhaps that's to be expected. The French law enforcement were pretty flat as characters, and there's just the general feeling that Ms. Christie hadn't quite gotten into the swing of it yet (which she did, of course, go on to do better than pretty much everyone). Seemed to go on a bit and felt convoluted. Hastings is also in peak ninny-Watson mode.