I can't think of any reason why it would be a negative
I'm not a dual citizen, but the only thing I've heard from others that might be considered a negative, is the higher taxes, as @Owain Taggart mentioned. I don't have a lot of experience in this area, but my understanding is that US taxes based on citizenship, whereas Canada taxes based on residency. So if you were dual and living in the US, you would only need to pay US taxes. But if you were dual and living in Canada, then you'd have to pay both Canadian and US taxes. (Apologies if my understanding is incorrect.)