It's longer because there's more to talk about.
Their chemistry? I... don't think so. XD Ai and Conan don't have anything in common besides being shrunk and soccer. And both of those are shallow similarities. They don't connect on a deeper level.
Weirdness? I can guarantee that if Ran and Haibara didn't exist everybody would say that AyuCon was canon. If you look at how many times he's grabbed her shoulders and held her hand and all of that, and completely forgot Haibara and Ran existed for a moment, you'd see them in a completely different light. I'm sure.
Realistic? AiCon? XD And Miss Mary Mack wasn't all dressed in black. xD lol In reality I seriously doubt that anybody would trust Haibara so quickly, and I doubt that Haibara would show her weaknesses to him so soon. And it may sound romantic, but it's not. That's just bad writing. xD
And they don't understand each other. xD I don't see why so many of you think Haibara has some sort of "understanding" of Conan that Ran and Ayumi don't have- if anything she's the farthest from understanding him. SHe questions half of the things he does. She understands why he does them, but she doesn't fully grasp what it really is that makes him the hero that he is. Ran and Ayumi can connect themselves a little closer than that because they WEREN'T raised in the environment Haibara was in. They didn't have to live by the rule "Fend for yourself and nobody else". (Her sister was an exception, yes, but Ayumi and Ran can understand the things he does like risking his life to save a total stranger.)
I'm so against it because Haibara doesn't need a boy who will back-sass her and just confuse her and argue with her and be against her on pretty much anything. And he doesn't need a woman like her being sarcastic about his choices or somebody he can't talk to about his day without boring her out of her mind. She needs somebody who will be sweet, delicate, yet be able to stand up to her and keep her soft when her darker side comes out. He needs a girl who can stand her own against him but still be gentle with him and be right there with him when he solves a case because she wants to solve it, too.
THAT is why I'm so against the pairing.