They do understand each other, I agree. They bring out the darkness in each other. They are both so full of hate and misery and anger and desire for POWER, but I think that lead them to loving the darkness in each other. I think Rumple lusted after the woman who was just as dark as he was and vice versa. He’s been alone (we assume) since Milah, all alone in his darkness, and he thought he craved more darkness. But what love really is, is finding the light amidst all the darkness. And together, there was nothing but dark between them. I don’t think the AU of Cora and Rumple would have been a good one. Regina may, in the end, have been enough for Cora when she got her heart, but I don’t think it would have worked that way with Rumple. Rumple and her feed each other the darkness and it would have been a never ending cycle.